Add a changelog to pytest-dev plugin requirements

[The-Compiler]

Since this just came up in pytest-dev/pytest-repeat#6 I propose adding a changelog to the requirements to submit a plugin to pytest-dev.

What do you think? As an user of plugins I really like to see what's changed between releases, see http://keepachangelog.com/ 😉

[nicoddemus]

I agree! Would you care to add a PR adding that? 😁

[The-Compiler]

I guess I could've done that from the beginning instead of an issue... 😆 done!

[nicoddemus]

Bad compiler! 😝

[RonnyPfannschmidt]

@The-Compiler @nicoddemus should we suggest restructuredtext as format?

[The-Compiler]

I thought about it, but I don't think it matters much. As a human reader, I really don't care what the underlying format is 😆

FWIW the page I linked which is pretty good suggests Markdown. I'd slightly prefer if it'd recommend ReST instead - but as said, I think it's fine.