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:

• Readable for each table (see docs)
• M2M (see docs)

[sinisaos]

Resolved by #555 and #1234