Expanding customization docs (#283)

* Expanding customization docs

As a new user I was very confused on how to either modify, override or create a new library. The existing docs were brief and had requirred knowledge that new users will not have,  and it took a lot of trial and error to make progress.

Also the wording seemed off. It kept talking about "client" when this project creates more than just clients, used "language" when some templates are not a language, and used library to mean a few seeminly different things. I've tried to consolidate the wording around "template" and "codegen".

The codegen is the Java code, and then you give it a "name" which is used for the template. That's the template folder it will live in, so it seems to work.

Feel free to use this as a base and edit it out, but this will all make a lot more sense to beginners. :)

docs/customization.md

@@ -1,38 +1,55 @@
 ## Customization
-### Modifying the client library format
 
-Don't like the default client syntax?  Want a different language supported?  No problem!  OpenAPI Generator processes mustache templates with the [jmustache](https://github.com/samskivert/jmustache) engine.  You can modify our templates or make your own.
+### Modifying a template
 
-You can look at `modules/openapi-generator/src/main/resources/${your-language}` for examples. To make your own templates, create your own files and use the `-t` flag to specify your template folder.  It actually is that easy.
+Clone OpenAPI Generator and take a look at the following directory: `modules/openapi-generator/src/main/resources/${template}`. In here you'll see all of the generators available, for most programming languages, web application frameworks and web servers. For example, if you are looking for the C# template, it's named `csharp`.
 
-### Making your own codegen modules
+Templates consist of multiple mustache files. [Mustache](https://mustache.github.io/) is used as the templating language for these templates, and the specific engine used is [jmustache](https://github.com/samskivert/jmustache).
 
-If you're starting a project with a new language and don't see what you need, openapi-generator can help you create a project to generate your own libraries:
+If you wish to modify one of these templates, copy and paste the template you're interested in to a templates directory you control. To let OpenAPI Generator know where this templates directory is, use the `-t` option (e.g: `-t ./templates/`).
+
+To tie that all together:
+
+```sh
+mkdir templates
+cp -r modules/openapi-generator/src/main/resources/${template} templates/
+java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate \
+  -t ./templates/ -g ruby -i ./foo.yml -o ./out/ruby
+```
+
+_**Note:** You cannot use this approach to create new templates, only override existing ones._
+
+### Creating a new template
+
+If none of the templates suit your needs at all, you can create a brand new template. OpenAPI Generator can help with this, using the `meta` command:
 
 ```sh
 java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar meta \
-  -o output/myLibrary -n myClientCodegen -p com.my.company.codegen
+  -o out/codegens/customCodegen -n myCodegen -p com.my.company.codegen
 ```
 
-This will write, in the folder `output/myLibrary`, all the files you need to get started, including a `README.md. Once modified and compiled, you can load your library with the codegen and generate clients with your own, custom-rolled logic.
+This will create a new directory `out/codegens/customCodegen`, with all the files you need to get started - including a `README.md`. Once modified and compiled, you can use your new codegen just like any other, with your own custom-rolled logic.
 
-You would then compile your library in the `output/myLibrary` folder with `mvn package` and execute the codegen like such:
+These names can be anything you like. If you are building a client for the whitespace language, maybe  you'd use the options `-o out/codegens/whitespace -n whitespace`. They can be the same, or different, it doesn't matter. The `-n` value will be become the template name.
+
+To compile your library, enter the `out/codegens/customCodegen` directory, run `mvn package` and execute the generator:
 
 ```sh
-java -cp output/myLibrary/target/myClientCodegen-openapi-generator-1.0.0.jar:modules/openapi-generator-cli/target/openapi-generator-cli.jar org.openapitools.codegen.OpenAPIGenerator
+java -cp out/codegens/customCodegen/target/myCodegen-openapi-generator-1.0.0.jar:modules/openapi-generator-cli/target/openapi-generator-cli.jar org.openapitools.codegen.OpenAPIGenerator
 ```
+
 For Windows users, you will need to use `;` instead of `:` in the classpath, e.g.
 ```
-java -cp output/myLibrary/target/myClientCodegen-openapi-generator-1.0.0.jar;modules/openapi-generator-cli/target/openapi-generator-cli.jar org.openapitools.codegen.OpenAPIGenerator
+java -cp out/codegens/customCodegen/target/myCodegen-openapi-generator-1.0.0.jar;modules/openapi-generator-cli/target/openapi-generator-cli.jar org.openapitools.codegen.OpenAPIGenerator
 ```
 
-Note the `myClientCodegen` is an option now, and you can use the usual arguments for generating your library:
+Note the `myCodegen` is an option for `-g` now, and you can use the usual arguments for generating your code:
 
 ```sh
-java -cp output/myLibrary/target/myClientCodegen-openapi-generator-1.0.0.jar:modules/openapi-generator-cli/target/openapi-generator-cli.jar \
-  io.openapitools.codegen.OpenAPIGenerator generate -g myClientCodegen\
+java -cp out/codegens/customCodegen/target/myCodegen-openapi-generator-1.0.0.jar:modules/openapi-generator-cli/target/openapi-generator-cli.jar \
+  io.openapitools.codegen.OpenAPIGenerator generate -g myCodegen \
   -i https://raw.githubusercontent.com/openapitools/openapi-generator/master/modules/openapi-generator/src/test/resources/2_0/petstore.yaml \
-  -o myClient
+  -o ./out/myClient
 ```
 
 
@@ -95,7 +112,6 @@ The ignore file allows for better control over overwriting existing files than t
 Examples:
 
 ```sh
-# OpenAPI Generator Ignore
 # Lines beginning with a # are comments
 
 # This should match build.sh located anywhere.