Create CONTRIBUTING.md (microsoft#1306)

Refactoring the contribution guide to a separate file.

CONTRIBUTING.md

@@ -0,0 +1,14 @@
+### Contributing
+Welcome, and thank you for your interest in contributing. There are many ways to contribute:
+- [Submit issues](https://github.com/Microsoft/vstest/issues) and help verify fixes as they are checked in.
+- Review the [source code changes](https://github.com/Microsoft/vstest/pulls).
+- [Contribute features and fixes](https://github.com/Microsoft/vstest-docs/blob/master/docs/contribute.md).
+- Contribute to the [documentation](https://github.com/Microsoft/vstest-docs).
+
+### Building
+If you want to understand how **vstest** works or want to debug an issue, you'll want to get the source, build it, and run the tool locally. **vstest** can be built from within Visual Studio or from the CLI.
+- [Building with Visual Studio](https://github.com/Microsoft/vstest-docs/blob/master/docs/contribute.md#building-with-visual-studio)
+- [Building with CLI, CI, Editors](https://github.com/Microsoft/vstest-docs/blob/master/docs/contribute.md#building-with-cli-ci-editors)
+
+### Thank You!
+Your contributions to open source, large or small, make projects like this possible. Thank you for taking the time to contribute.