Specify downgrade process (SYN-625)

[matrixbot]

Submitted by @​kegan:matrix.org
It would be nice to have a set of instructions for people who want to downgrade to non-broken versions. Caerdwyn wanted to do this, and I couldn't give a complete answer because I don't know exactly how one would go about downgrading. Specifying this in the synapse README would be really useful.

(Imported from https://matrix.org/jira/browse/SYN-625)

[richvdh]

I wrote a few bits in #6050