Show guidelines when creating a new thread in announce

[wilzbach]

E.g.

• what should be in announce (it happens quite often that a post should have been in general)
• tips about how to write a good annoucement (there have been many articles that were either too short or too long)
• advice how to promote your new tool, library etc.

https://dlang.org/howto-promote.html (this page is probably unknown to most devs)

[wilzbach]

This is probably also a good solution to #62, #63, #64, and #65 ;-)