Create CONTRIBUTING.md

CONTRIBUTING.md

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+## Before submitting an issue
+
+ - Before submitting an issue, search the [open issue](https://github.com/swagger-api/swagger-codegen/issues) and [closed issue](https://github.com/swagger-api/swagger-codegen/issues?q=is%3Aissue+is%3Aclosed) to ensure no one else has reported something similar before.
+ - The issue should contain details on how to repeat the issue, e.g. 
+   - the Swagger spec for reproducing the issue (:bulb: use [Gist](https://gist.github.com) to share). If the Swagger spec cannot be shared publicly, it will be hard for the community to help
+   - version of Swagger Codegen
+   - language (`-l` in the command line, e.g. java, csharp, php)
+ - You can also make a suggestion or ask a question by opening an "issue"
+
+## Before submitting a PR
+
+ - Search the [open issue](https://github.com/swagger-api/swagger-codegen/issues) to ensure no one else has reported something similar and no one is actively working on similar proposed change.
+ - If no one has suggested something similar, open an ["issue"](https://github.com/swagger-api/swagger-codegen/issues) with your suggestion to gather feedback from the community.
+
+## How to contribute
+
+### Code generators
+
+All the code generators can be found in [modules/swagger-codegen/src/main/java/io/swagger/codegen/languages](https://github.com/swagger-api/swagger-codegen/tree/master/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages)
+
+### Templates
+
+All the templates ([mustache](https://mustache.github.io/)) can be found in [modules/swagger-codegen/src/main/resources](https://github.com/swagger-api/swagger-codegen/tree/master/modules/swagger-codegen/src/main/resources).
+
+For a list of variables available in the template, please refer to this [page](https://github.com/swagger-api/swagger-codegen/wiki/Mustache-Template-Variables)
+
+
+### Style guide
+Code change should conform to the programming style guide of the respective langauages:
+- C#: https://msdn.microsoft.com/en-us/library/vstudio/ff926074.aspx
+- Java: https://google.github.io/styleguide/javaguide.html
+- ObjC: https://github.com/NYTimes/objective-c-style-guide
+- PHP: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md
+- Python: https://www.python.org/dev/peps/pep-0008/
+- Ruby: https://github.com/bbatsov/ruby-style-guide
+- TypeScript: https://github.com/Microsoft/TypeScript/wiki/Coding-guidelines
+
+
+For other languages, feel free to suggest.
+
+You may find the current code base not 100% conform to the coding style and we welcome contributions to fix those.
+
+### Testing
+
+To add test cases (optional) covering the change in the code generator, please refer to [modules/swagger-codegen/src/test/java/io/swagger/codegen](https://github.com/swagger-api/swagger-codegen/tree/master/modules/swagger-codegen/src/test/java/io/swagger/codegen)
+
+To test the templates, please perform the following:
+- Update the [Petstore](http://petstore.swagger.io/) sample by running the shell script under `bin` folder. For example, run `./bin/ruby-petstore.sh` to update the Ruby PetStore API client under [`samples/client/petstore/ruby`](https://github.com/swagger-api/swagger-codegen/tree/master/samples/client/petstore/ruby) (For Windows, the batch files can be found under `bin\windows` folder)
+- Run the tests in the sample folder, e.g. in `samples/client/petstore/ruby`, run `mvn integration-test -rf :RubyPetstoreClientTests`. (some languages may not contain unit testing for Petstore and we're looking for contribution from the community to implement those tests)
+- Finally, git commit the updated samples files: `git commit -a`
+  (`git add -A` if added files with new test cases)
+
+To start the CI tests, you can run `mvn verify -Psamples` (assuming you've all the required tools installed to run tests for different languages) or you can leverage http://travis-ci.org to run the CI tests by adding your own Swagger-Codegen repository.
+
+### Tips
+- Smaller changes are easier to review
+- [Optional] For bug fixes, provide a Swagger spec to repeat the issue so that the reviewer can use it to confirm the fix
+- Add test case(s) to cover the change
+- Document the fix in the code to make the code more readable
+- Make sure test cases passed after the change (one way is to leverage https://travis-ci.org/ to run the CI tests)