Add a line instructing users to include media screenshots (#7790)

Making media up for release notes / tweets is becoming very time
consuming. I spoke to Max and suggested we ask users to submit media for
their features, to reduce what we need to produce for tweets and such. I
dont know if this is the best way to signal it; I don't like adding more
to the PR template, but I'm not sure of a better way at the moment.


Release Notes:

- N/a

.github/pull_request_template.md

@@ -7,3 +7,5 @@ Release Notes:
 **or**
 
 - N/A
+
+Optionally, include screenshots / media showcasing your addition that can be included in the release notes.