Create contribution guidelines.

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CONTRIBUTING.md

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+Contributing
+------------
+
+You’ve decided to contribute? Awesome! Here are some things you should know.
+
+- The `master` branch hosts the latest stable version.
+- The development version is in the `integration` branch.
+
+#### The lifecycle of a great contribution
+
+1. Fork the repo.
+
+2. Create a new branch off of `integration` with a descriptive name like
+   `feature/add-more-emoji`.
+
+3. Make [atomic commits][atomic], i.e. commits that only modify a single aspect
+   of the project. This makes it easier to review, apply and roll back changes.
+
+4. [Write good commit messages.][messages] Here’s an example, adapted from Tim
+   Pope’s example and [git-scm.com][git-scm]:
+
+   ```
+   The first line should fit in 50 characters.
+
+   More detailed explanatory text, if necessary. Wrap it to about 72
+   characters or so.
+
+   It’s a good idea to use the imperative present tense when writing the
+   first line. In other words, use commands. Instead of "I added tests for"
+   or "Adding tests for," use "Add tests for."
+
+   - Bullet points are okay, too.
+
+   - Typically a hyphen or asterisk is used for the bullet, preceded by a
+     single space, with blank lines in between, but conventions vary here.
+
+   ```
+
+5. If you need to clean up your commit history before submitting a pull request,
+   use [interactive rebase][rebase]: `git rebase -i integration`. This is good
+   for combining commits that fix mistakes in previous ones.
+
+6. When done, open a pull request from your feature branch to the `integration`
+   branch. Be sure to select `integration` instead of `master`.
+
+[atomic]:   https://en.wikipedia.org/wiki/Atomic_commit#Atomic_Commit_Convention
+[messages]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+[git-scm]:  http://git-scm.com/book/ch5-2.html
+[rebase]:   https://help.github.com/articles/about-git-rebase/