Adds contributing guide.

So users know how to contribute to the project so their
input has a high chance of being accepted upstream.

Author: jakubpawlowicz

CONTRIBUTING.md

@@ -0,0 +1,49 @@
+# Bug reports
+
+Every piece of software has bugs, and squashing them should always be the priority.
+
+If you spotted an incorrect behavior, knowing the following facts will help fixing it:
+
+* What's your input CSS and expected output?
+* What options do you pass to `cleancss`?
+* What version of clean-css-cli do you use?
+* What operating system do you use?
+
+# Pull requests
+
+We love pull requests! To contribute to clean-css first fork, then clone the repo:
+
+```shell
+git clone git@github.com:your-username/clean-css-cli.git
+```
+
+Make sure you have node 4.0+ installed so npm can download all dependencies for you:
+
+```shell
+npm install
+```
+
+Make sure the tests pass:
+
+```shell
+npm test
+```
+
+Then add tests for your change. Check if tests fail. Make your change. Make the tests pass.
+
+At the end make sure code styling validation passes:
+
+```shell
+npm run check
+```
+
+Finally push to your fork and [submit a pull request](https://github.com/jakubpawlowicz/clean-css-cli/compare/).
+
+At this point you're waiting for a PR review which should not thake more than a day.
+
+Some things that will increase the chance that your pull request is accepted:
+
+* Write tests.
+* Write self-documenting code.
+* Squash commits.
+* Write a [good commit message](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).