Create CONTRIBUTING.md

Author: Caitlin Bales (MSFT)

CONTRIBUTING.md

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+# Contributing to the Microsoft Graph SDK for Java
+
+The Microsoft Graph SDK is available for all manner of contribution. There are a couple of different recommended paths to get contributions into the released version of this SDK.
+
+__NOTE__ A signed a contribution license agreement is required for all contributions, and is checked automatically on new pull requests. Please read and sign the agreement https://cla.microsoft.com/ before starting any work for this repository.
+
+## File issues
+
+The best way to get started with a contribution is to start a dialog with the owners of this repository. Sometimes features will be under development or out of scope for this SDK and it's best to check before starting work on contribution.
+
+## Submit pull requests for trivial changes
+
+If you are making a change that does not affect the interface components and does not affect other downstream callers, feel free to make a pull request against the __dev__ branch.  The dev branch will be updated frequently.
+
+Revisions of this nature will result in a 0.0.X change of the version number.
+
+## Submit pull requests for features
+
+If major functionality is being added, or there will need to be gestation time for a change, it should be submitted against the __feature__ branch.
+
+Revisions of this nature will result in a 0.X.X change of the version number.
+
+## Submit pull requests for model or request files
+You can revise code in the extension folders of these folders. See [Extending the Library](https://github.com/microsoftgraph/msgraph-sdk-java/wiki/Extending-the-Library) for more information on how to work with the generated parts of the library.