Adding a CONTRIBUTING.md file

CONTRIBUTING.md

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+Thanks for contributing to `nvm`!
+
+We love pull requests, they're our favorite.
+
+However, before submitting, please review the following:
+
+ - Please include tests. Changes with tests will be merged very quickly.
+ - Please manually confirm that your changes, and all tests, pass in `bash`, `sh`, and `zsh`. We hope to soon automatically run all these tests, but in the meantime, we have to rely on each contributor doing so.
+ - Please maintain consistent whitespace - 2-space indentation, trailing newlines in all files, etc.
+ - Any time you make a change to your PR, please rebase freshly on top of master. Nobody likes merge commits.
+
+Even if you don't have all of these items covered, please still feel free to submit a PR! Someone else may be inspired and volunteer to complete it for you.
+
+Thanks again!
+