This is essentially an implementation of the proposal made at https://hackmd.io/@richvdh/BJYXQMQHO, though the details have ended up looking slightly different.
The hope here is that by moving all the schema files into synapse/storage/schema, it gets a bit easier for newcomers to navigate.
It certainly got easier for me to write a helpful README. There's more to do on that front, but I'll follow up with other PRs for that.