Add minimal Contributor guidelines

CONTRIBUTING.adoc

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+= Contributing to Spring Session
+
+Spring Session is released under the Apache 2.0 license. If you would like to contribute
+something, or simply want to hack on the code this document should help you get started.
+
+
+
+== Using GitHub issues
+
+We use GitHub issues to track bugs and enhancements. If you have a general usage question
+please ask on http://stackoverflow.com[Stack Overflow]. The Spring Session team and the
+broader community monitor the http://stackoverflow.com/tags/spring-session[`spring-session`]
+tag.
+
+If you are reporting a bug, please help to speed up problem diagnosis by providing as much
+information as possible. Ideally, that would include a small sample project that
+reproduces the problem.
+
+
+
+== Sign the Contributor License Agreement
+Before we accept a non-trivial patch or pull request we will need you to sign the
+https://support.springsource.com/spring_committer_signup[contributor's agreement].
+If you have previously signed the CLA for any project, there is no need to sign up again.
+Signing the contributor's agreement does not grant anyone commit rights to the main
+repository, but it does mean that we can accept your contributions, and you will get an
+author credit if we do.  Active contributors might be asked to join the core team, and
+given the ability to merge pull requests. Use the project `Spring Security` (the parent project)
+and '`Rob Winch`' in the project lead field when you complete the form.
+