docs: add 20.2 planning

/cc @dbaker @evelikov @tanty @jasuarez any volunteers?

I'm happy to do it if nobody else wants to, but I think there's value in not having the same maintainer do two releases in a row; for starters, it makes sure there's communication about things that can have changed in the process from one release to the next, so that things don't just stay in the previous release's maintainer's head :)