add roadmap

[plebe1us]

We should be clear about what core features are missing, possibly right in the 5chan homepage. A roadmap could also motivate contributors.

For example: anon mode is missing but it's theoretically possible (https://t.me/fivechandev/9743), someone has to figure out how to add post numbers, board voting (pubsub voting) is an essential core feature that is possible and planned but not documented yet, and we have to explain how media upload could work even on web (with a browser extension, or a list of centralized image hosting providers that give us CORS access, or public RPC-run user profile nodes that would selfhost media on IPFS assuming they'd be well moderated).