Add contributing guide

CONTRIBUTING.md

+Paranoia is an open source project and we encourage contributions.
+
+## Filing an issue
+
+When filing an issue on the Paranoia project, please provide these details:
+
+* A comprehensive list of steps to reproduce the issue.
+* What you're *expecting* to happen compared with what's *actually* happening.
+* Your application's complete `Gemfile.lock`, and `Gemfile.lock` as text in a [Gist](https://gist.github.com) (*not as an image*)
+* Any relevant stack traces ("Full trace" preferred)
+
+In 99% of cases, this information is enough to determine the cause and solution
+to the problem that is being described.
+
+Please remember to format code using triple backticks (\`) so that it is neatly
+formatted when the issue is posted.
+
+## Pull requests
+
+We gladly accept pull requests to add documentation, fix bugs and, in some circumstances,
+add new features to Paranoia.
+
+Here's a quick guide:
+
+1. Fork the repo.
+
+2. Run the tests. We only take pull requests with passing tests, and it's great
+to know that you have a clean slate.
+
+3. Create new branch then make changes and add tests for your changes. Only
+refactoring and documentation changes require no new tests. If you are adding
+functionality or fixing a bug, we need tests!
+
+4. Push to your fork and submit a pull request.