Add compatibility note to CONTRIBUTING

Closes #72

Author: peterh

.github/CONTRIBUTING.md

@@ -7,3 +7,9 @@ If you are opening a feature request, you are implicitly volunteering to
 implement that feature. Obvious feature requests should be made via a pull
 request. Complex feature requests will be interpreted as a
 request-for-comments, and will be closed once comments are given.
+
+#### Liner must remain backwards compatible
+
+The API of Liner must not change in an incompatible way. When making
+changes to liner, please use the [Go 1 Compatibility Promise](https://golang.org/doc/go1compat)
+as a guideline.