Create Contributing.md (#668)

Co-authored-by: Avery-Dunn <[email protected]>

Author: bgavrilMS

Contributing.md

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+# CONTRIBUTING
+
+Microsoft Authentication Library for Java welcomes new contributors.  This document will guide you
+through the process.
+
+## CONTRIBUTOR LICENSE AGREEMENT
+
+Please visit [https://cla.microsoft.com/](https://cla.microsoft.com/) and sign the Contributor License
+Agreement.  You only need to do that once. We can not look at your code until you've submitted this request.
+
+
+## Build
+
+Use Java8.
+
+## Test
+
+Unit tests should run as expected. Integration tests require certificate / secrets which are deployed on CI. External contributors are not able to run integration tests manually.