Added CONTRIBUTING.md

Author: Nakul Sabharwal

CONTRIBUTING.md

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+# Contributing to the Microsoft Graph Core SDK for Java
+
+The Microsoft Graph Core SDK for Java 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.
+
+## Add yourself as a contributor
+
+This project follows the [all contributors](https://github.com/kentcdodds/all-contributors) specification. When making a contribution, please add yourself to the table of contributors:
+
+ 1. In section 5. of the [README.md](https://github.com/microsoftgraph/msgraph-sdk-java-core/blob/dev/readme.md), after the last "|", copy and paste a new blank contributor element
+    ```html
+    [<img src="https://avatars.githubusercontent.com/u/<your-uid>?v=4" width="100px;"/><br />
+    <sub><b>Your Name</b></sub>](your website or github page)<br />
+    [emoji](link "alt-text") |
+    ```
+
+    You can get your GitHub UID by inspecting your GitHub avatar image.
+
+2. For each contribution type (see [emoji key](https://github.com/kentcdodds/all-contributors#emoji-key) for a list of contribution types), add an emoji and a relevant link and alt-text.
+    
+    For example, if you write a blogpost on how to use the SDK, you would include:
+    
+	```html
+	    [??]("https://myblog.com/using-the-java-sdk" "Blog Post")