Added missing CONTRIBUTING file

Author: WyriHaximus

CONTRIBUTING.md

@@ -0,0 +1,38 @@
+# Contributing
+
+Pull requests are highly appreciated. Here's a quick guide.
+
+Fork, then clone the repo:
+
+    git clone [email protected]:your-username/psr7-oauth1.git
+
+Set up your machine:
+
+    composer install
+
+Make sure the tests pass:
+
+    make unit
+
+Make your change. Add tests for your change. Make the tests pass:
+
+    make unit
+    
+Before committing and submitting your pull request make sure it passes PSR2 coding style, unit tests pass and pass on all supported PHP versions:
+
+    make contrib
+
+Push to your fork and [submit a pull request][pr].
+
+[pr]: https://help.github.com/articles/creating-a-pull-request/
+
+At this point you're waiting on me. I like to at least comment on pull requests
+within a day or two. I may suggest some changes or improvements or alternatives.
+
+Some things that will increase the chance that your pull request is accepted:
+
+* Write tests.
+* Follow PSR2 (travis will also check for this).
+* Write a [good commit message][commit].
+
+[commit]: http://chris.beams.io/posts/git-commit/