add minimal contributing file

docs/contributing.md

@@ -0,0 +1,22 @@
+# Contributing to imageproxy
+
+## Types of contributions
+
+Simple bug fixes for existing functionality are always welcome.  In many cases,
+it may be helpful to include a reproducible sample case that demonstrates the
+bug being fixed.
+
+For new functionality, it's general best to open an issue first to discuss it.
+
+## Contributor License Agreement
+
+Even though this is effectively a personal project of mine, it is still governed
+by Google's Contributor License Agreement because of my employment there.  You
+(or your employer) retain the copyright to your contribution; the CLA simply
+gives permission to use and redistribute your contributions as part of the
+project.  Head over to <https://cla.developers.google.com/> to see your current
+agreements on file or to sign a new one.
+
+You generally only need to submit a CLA once, so if you've already submitted one
+(even if it was for a different Google project), you probably don't need to do
+it again.