add license and other guidelines

Author: rtfpessoa

CHANGELOG.md

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+### July 16, 2014 - v0.1.0
+
+* Initial release (Rodrigo Fernandes)

CONTRIBUTING.md

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+# Contributing
+
+One of the great things about open source projects is that anyone can contribute code.
+bitbucket-scala-client is a small project and we want to encourage everyone to submit their
+patches freely. To help you in that process, there are several things that you should keep in mind.
+
+## Use Pull Requests
+
+If you want to submit code, please use a GitHub pull request.
+This is the fastest way for us to evaluate your code and to merge it into the code base.
+Please don't file an issue with snippets of code. Doing so means that we need to manually
+merge the changes in and update any appropriate tests. That decreases the likelihood that
+your code is going to get included in a timely manner. Please use pull requests.
+
+## Unit Tests
+
+Regardless of the changes you are making, please make sure that your pull request includes
+appropriate unit tests.
+
+## Testing
+
+Along with submitting unit tests, please make sure that you have used the built-in
+testing capabilities in the bitbucket-scala-client build system. At a minimum, you should
+run `activator test`. That generates all of the built files and runs all of the unit tests
+in the project.