Update contribution guide to indicate response time.

CONTRIBUTING.md

@@ -22,6 +22,16 @@ use GitHub pull requests for this purpose.
 Please read the [style guide](docs/style.md) for tips on the project coding
 guidelines.
 
+### Response Time
+
+This repository is maintained as a best effort service.
+
+Response times to issues and PRs may vary with the availability of the
+maintainers.  We appreciate your patience.
+
+PRs with unit tests will be merged promptly.  All other requests (issues and
+PRs) may take longer to be responded to.
+
 ### The small print
 Contributions made by corporations are covered by a different agreement than
 the one above, the Software Grant and Corporate Contributor License Agreement.