Add the towncrier bot for remembering release notes

[TomNicholas]

darn I forgot release notes! We really need that towncrier bot.

Originally posted by @maxrjones in #640 (comment)

[TomNicholas]

I think this is the bot @maxrjones was referring to: https://github.com/scientific-python/action-towncrier-changelog.

Not sure if it assumes that each of your release notes goes into a separate file though.