raindrops 0.20.0

Raindrops may now use file-backed mmap() rather than anonymous
memory.  Thanks to KJ Tsanaktsidis for the patch:

https://yhbt.net/raindrops-public/20211125065618.3432-1-ktsanaktsidis@zendesk.com/T/

The documentation is also updated to note our mail archives are now
available via IMAP(S).