Add contributing file

Author: simeg

CONTRIBUTING.md

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+# Contributing
+
+:raised_hands::tada: First off, thanks for taking the time to contribute! :tada::raised_hands:
+
+The following is a set of guidelines for contributing to react-datetime. The purpose of these guidelines is to maintain a high quality of code *and* traceability. Please respect these guidelines.
+
+## General
+This repository use tests and a linter as automatic tools to maintain the quality of the code. These two tasks are run locally on your machine before every commit (as a pre-commit git hook), if any test fail or the linter gives an error you cannot create the commit. They are also run on a Travis CI machine when you create a pull request, and will not be merged unless Travis says all tests and the linting pass.
+
+## Git Commit Messages
+* Use the present tense ("Add feature" not "Added feature")
+* Use the imperative mood ("Move cursor to..." not "Moves cursor to...")
+  * Think of it as you are *commanding* what your commit is doing
+  * Git itself uses the imperative whenever it creates a commit on your behalf, so it makes sense for you to use it too!
+* Use the body to explain *what* and *why*
+  * If the commit is non-trivial, please provide more detailed information in the commit body message
+  * *How* you made the change is visible in the code and is therefore very rarely necessary to include in the commit body message, but *why* you made the change is often harder to guess and should therefore be included in the commit body message
+
+[Here's a nice blog post](http://chris.beams.io/posts/git-commit/) about how to write great git messages - it's worth a read.
+
+## Pull Requests
+* Follow the current code style
+* Write tests for your changes
+* Add to documentation if it is needed
+* End files with a newline
+