Document guidelines for writing release notes

[axelboc]

As suggested in #683 (reply in thread)

[aplaice]

Thanks so much! This is very clear and I finally understand exactly what all the emojis mean (some were completely obvious, but for some (e.g. 👉) I sort-of guessed based on context in the previous release notes).

(A couple of minor type edit: typo 🤦 fixes.)

[axelboc]

Glad it brings some clarity 😊 - and good catch on all the typos!