Add CONTRIBUTING guidelines

Stolen from spree/spree

CONTRIBUTING.md

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+Ransack is an open source project and we encourage contributions.
+
+## Filing an issue
+
+When filing an issue on the Ransack project, please provide these details:
+
+* A comprehensive list of steps to reproduce the issue.
+* The version of Ransack *and* the version of Rails.
+* 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.
+
+Any issue that is open for 14 days without actionable information or activity will be marked as "stalled" and then closed. Stalled issues can be re-opened if the information requested is provided.
+
+## Pull requests
+
+We gladly accept pull requests to fix bugs and, in some circumstances, add new features to Ransack.
+
+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:
+
+        $ bundle install
+        $ bundle exec rake test_app
+        $ bundle exec rake
+
+3. Add a test for your change. Only refactoring and documentation changes
+require no new tests. If you are adding functionality or fixing a bug, we need
+a test!
+
+4. Make the test pass.
+
+5. Push to your fork and submit a pull request. If the changes will apply cleanly to the latest stable branches and master branch, you will only need to submit one pull request.
+
+At this point you're waiting on us. We like to at least comment on, if not
+accept, pull requests within three business days (and, typically, one business
+day). We may suggest some changes or improvements or alternatives.
+
+Some things that will increase the chance that your pull request is accepted,
+taken straight from the Ruby on Rails guide:
+
+* Use Rails idioms and helpers
+* Include tests that fail without your code, and pass with it
+* Update the documentation, the surrounding one, examples elsewhere, guides,
+  whatever is affected by your contribution
+
+Syntax:
+
+* Two spaces, no tabs.
+* No trailing whitespace. Blank lines should not have any space.
+* Prefer &&/|| over and/or.
+* `MyClass.my_method(my_arg)` not `my_method( my_arg )` or my_method my_arg.
+* `a = b` and not `a=b`.
+* `a_method { |block| ... }` and not `a_method { | block | ... }`
+* Follow the conventions you see used in the source already.
+
+And in case we didn't emphasize it enough: we love tests!