chore: add CONTRIBUTING.md

CONTRIBUTING.md

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+Contributing
+============
+
+If you wish to contribute please read the following quick guide.
+
+# Want a Feature?
+You can request a new feature by submitting an issue. If you would like to implement a new feature feel free to issue a
+Pull Request.
+
+
+# Pull requests (PRs)
+PRs are awesome. However, before you submit your pull request consider the following guidelines:
+
+ - Search GitHub for an open or closed Pull Request that relates to your submission. You don't want to duplicate effort.
+ - When issuing PRs that change code, make your changes in a new git branch based on master:
+
+   ```bash
+   git checkout -b my-fix-branch master
+   ```
+
+ - Documentation (i.e: README.md) changes can be made directly against master.
+ - Run the full test suite before submitting and make sure all tests pass (obviously =P).
+ - Try to follow our [**coding style rules**](https://github.com/showdownjs/code-style/blob/master/README.md).
+   Breaking them prevents the PR to pass the tests.
+ - Refrain from fixing multiple issues in the same pull request. It's preferable to open multiple small PRs instead of one
+   hard to review big one. Also, don't reuse old forks (or PRs) to fix new issues.
+ - If the PR introduces a new feature or fixes an issue, please add the appropriate test case.
+ - We use commit notes to generate the changelog. It's extremely helpful if your commit messages adhere to the
+ [**AngularJS Git Commit Guidelines**](https://github.com/showdownjs/code-style/blob/master/README.md#commit-message-convention).
+ - If we suggest changes then:
+   - Make the required updates.
+   - Re-run the Angular test suite to ensure tests are still passing.
+   - Rebase your branch and force push to your GitHub repository (this will update your Pull Request):
+
+   ```bash
+   git rebase master -i
+   git push origin my-fix-branch -f
+   ```
+ - After your pull request is merged, you can safely delete your branch.
+
+If you have time to contribute to this project, we feel obliged that you get credit for it.
+These rules enable us to review your PR faster and will give you appropriate credit in your GitHub profile.
+We thank you in advance for your contribution!
+
+
+# Joining the team
+We're looking for members to help maintaining Showdown.
+Please see [this issue](https://github.com/showdownjs/showdown/issues/114) to express interest or comment on this note.