Provide README.md for tsnet

[WalterHub]

Provide a README.md to explain the purpose of tsnet, how to get started with it, and to provide additional guidance.

[WalterHub]

Folder is https://github.com/tailscale/tailscale/tree/main/tsnet.

[bradfitz]

We already have:

https://tailscale.com/docs/features/tsnet
and
https://pkg.go.dev/tailscale.com/tsnet

A README.md is fine to add, but let's not make it a third thing we need to manually sync.

Let's instead auto-generate the README.md from the godoc like we do in corp with misc/genreadme.

Then it's less ongoing sync maintenance, and it also makes https://pkg.go.dev/tailscale.com/tsnet better for people who end up there.