Add info about how to write migration guides to contributor book

[JasmineLowen]

Yesterday I wanted to write a migration guide for one of my PRs. I didn't know how to, so initially (naturally) I was looking at https://bevy.org/learn/contribute/introduction/. However there is no info about the migration guides creation there, so I had to ask on discord.

It would be nice to have a very brief section in either

• https://bevy.org/learn/contribute/helping-out/opening-pull-requests/
• https://bevy.org/learn/contribute/helping-out/writing-docs/