Add a CONTRIBUTING.md starting point

As that seems nice, and I wanted there to be a (subtle) pointer to
the ideas page.

CONTRIBUTING.md

@@ -0,0 +1,25 @@
+
+Gabbi gets better because of the contributions from the people who use
+it. These contributions come in many forms:
+
+* Joining the #gabbi channel on Freenode IRC at freenode.net and
+  having a chat.
+* Improvements to make the documentation more complete, more correct
+  and typo free.
+* Reporting and reviewing bugs in the
+  [issues](https://github.com/cdent/gabbi/issues)
+* Providing [pull requests](https://github.com/cdent/gabbi/pulls)
+  containing fixes and new features.
+
+If you have an idea for a new feature it is best to review the
+[Ideas](https://github.com/cdent/gabbi/wiki/Ideas) wiki page and the
+existing issues and pull requests to see if there is existing work you
+can contribute to. It's also worthwhile to ask around in IRC.
+
+In general the default stance with gabbi is to avoid adding new features
+if we can come up with some way to use the existing features to solve
+the requirements of your tests. This helps to keep the test format
+as clean and readable as possible.
+
+If you reach an impasse, create an issue and provide as much info as you
+can about your situation and together we can try to figure it out.