Update pull request template with authorship info

People want to add themselves to AUTHORS. That's fine, but it's not
enough as it also needs to be added to NICKS and script/authors.go needs
to be run. I'd rather have us do this and do it correctly so lets
document that people should not worry about it.

PULL_REQUEST_TEMPLATE.md

@@ -13,3 +13,14 @@ if new tests are not included.
 
 If this is a GUI change, include screenshots of the change. If not, please
 feel free to just delete this section.
+
+### Authorship
+
+Every author of a code contribution (Go, Javascript, HTML, CSS etc, with the
+possible exception of minor typo corrections and similar) is recorded in the
+AUTHORS and NICKS files and the in-GUI credits. If this is your first
+contribution, a maintainer will add you properly before accepting the
+contribution. You need not do so yourself or worry about the fact that the
+"authors" automated test fails. However, if your name (such as you want it
+presented in the credits) is not visible on your Github profile or in your
+commit messages, please assist by providing it here.