Clarify the Javascript README for local use (#6044)

- Add instructions for installing the generated Javascript library locally without publishing to a remote npm registry.
- Clarify what `main.js` is.

modules/swagger-codegen/src/main/resources/Javascript/README.mustache

@@ -31,6 +31,30 @@ Then install it via:
 npm install {{{projectName}}} --save
 ```
 
+##### Local development
+
+To use the library locally without publishing to a remote npm registry, first install the dependencies by changing 
+into the directory containing `package.json` (and this README). Let's call this `JAVASCRIPT_CLIENT_DIR`. Then run:
+
+```shell
+npm install
+```
+
+Next, [link](https://docs.npmjs.com/cli/link) it globally in npm with the following, also from `JAVASCRIPT_CLIENT_DIR`:
+
+```shell
+npm link
+```
+
+Finally, switch to the directory you want to use your {{{projectName}}} from, and run:
+
+```shell
+npm link /path/to/<JAVASCRIPT_CLIENT_DIR>
+```
+
+You should now be able to `require('{{{projectName}}}')` in javascript files from the directory you ran the last 
+command above from.
+
 #### git
 #
 If the library is hosted at a git repository, e.g.
@@ -45,7 +69,8 @@ then install it via:
 
 The library also works in the browser environment via npm and [browserify](http://browserify.org/). After following
 the above steps with Node.js and installing browserify with `npm install -g browserify`,
-perform the following (assuming *main.js* is your entry file):
+perform the following (assuming *main.js* is your entry file, that's to say your javascript file where you actually 
+use this library):
 
 ```shell
 browserify main.js > bundle.js