add contribution guidelines

CONTRIBUTING.md

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+# Contributing to criticalCSS
+
+Contributions are appreciated. In order for us to consider including a contribution, it does have to meet a few criteria:
+
+* Code is specific to one issue (eg. feature, extension or bug)
+* Code is formatted according to JavaScript Style Guide.
+* Code has full test coverage and all tests pass.
+
+## Code to an Issue
+
+Use a separate git branch for each contribution. Give the branch a meaningful name.
+When you are contributing a new extensions use the name of this extension, like `dom-toggleclass`.
+Otherwise give it a descriptive name like `doc-generator` or reference a specific issue like `issues-12`.
+When the issue is resolved create a pull request to allow us to review and accept your contribution.
+
+## JavaScript Style Guide
+
+Code should be formatted according to the [jQuery JavaScript Style Guide](http://contribute.jquery.org/style-guide/).