contributing: add notes on describing contributions [ci skip]
Adds a couple of blurbs on how to describe contributions to the git
practice 'contributing' section.
@@ -51,6 +51,19 @@ email them to maintainers, or request a maintainer pull from your tree directly
-- but note that some maintainers will be more receptive to these methods than
others.
+When contributing changes, via whatever means, make sure you describe them
+appropriately. You should attach a reasonably high-level summary of what the
+changes are and what they do; reference any useful background material that may
+exist, e.g. a GitHub issue, a mailing list discussion, a UP, etc. [Here][jbpr]
+is a good example of a pull request with a useful, concise description.
+
+If your changes replace significant extant functionality, be sure to compare
+them with the thing you're replacing. You may also want to cc maintainers,
+reviewers, or other parties who might have a particular interest in what you're
+contributing.
+
+[jbpr]: https://github.com/urbit/urbit/pull/1782
+
### Hygiene
Commits should generally be relevant, atomic, and have descriptions formatted
