Add contribution guidelines

CONTRIBUTING.md

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+# How to contribute
+
+Third-party patches are essential for keeping mongoid-autoinc great.
+We want to keep it as easy as possible to contribute changes that get things working in your environment.
+There are a few guidelines that we need contributors to follow so that we can have a chance of keeping on top of things.
+
+Follow the styleguide when changing code:
+[80beans Styleguide](https://gist.github.com/suweller/b896eb9e66fc6ab3640d)
+
+## Making Changes
+
+* Create an issue so we can discuss the patch and prevent duplication of effort.
+* Create a topic branch from where you want to base your work.
+  * This is usually the `master` branch.
+  * Only target release branches if you are certain your fix must be on that
+    branch.
+  * To quickly create a topic branch based on master; `git branch
+    my_contribution master` then checkout the new branch with `git
+    checkout my_contribution`.  Please avoid working directly on the
+    `master` branch.
+* Make commits of logical units.
+* Check for unnecessary whitespace with `git diff --check` before committing.
+* Make sure your commit messages are in the proper format.
+
+```
+    Clarify CONTRIBUTING with an example
+
+    Without this patch applied the example commit message in the CONTRIBUTING
+    document is not a concrete example.  This is a problem because the
+    contributor is left to imagine what the commit message should look like
+    based on a description rather than an example.  This patch fixes the
+    problem by making the example concrete and imperative.
+
+    The first line is a real life imperative statement with a ticket number
+    from our issue tracker.  The body describes the behavior without the patch,
+    why this is a problem, and how the patch fixes the problem when applied.
+
+    Closes issue #11
+```
+
+* Reference the issue created earlier like in the above example commit.
+* Make sure you have added the necessary tests for your changes.
+* Run _all_ the tests to assure nothing else was accidentally broken.
+* Create a pull-request.