adding Contributing.md

CONTRIBUTING.md

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+# Contributing
+
+We welcome contributions. Great ways to contribute include trying things out, filing bugs, joining in design conversations and fixing issues. If you're looking at places to contribute code, take a look at our [up-for-grabs issues](https://github.com/dotnet/roslyn-sdk/issues?q=is%3Aopen+is%3Aissue+label%3A%22Up+for+Grabs%22).
+
+If you want to submit a feature or a substantial code contribution, please discuss it first with the the team by [filing an issue](https://github.com/dotnet/roslyn-sdk/issues/new).