Add `readable` and `M2M` examples to playground

[dantownsend]

When I'm experimenting with Piccolo queries I typically use the playground:

piccolo playground run

I try and make the schema used by the playground as fully featured as possible, so I can experiment with most Piccolo features.

Some features the playground schema doesn't have which need adding:

• [x] Readable for each table (see docs)
• [ ] M2M (see docs)