Docs: suggest creating an empty key

[stchris]

While setting up locally (with the sample_env you provide) it isn't possible to create an account if the SIGNATURE_PUBKEY_PEM file isn't there:

thread 'tokio-runtime-worker' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/ap.rs:45:21

Since I don't care about federation I side-stepped this with mkdir local && touch local/public.pem. You might want to mention this in the docs (or handle the error differently).

[alexwennerberg]

Going to fail gracefully if this key is absent with unwrap_or("".to_owned())

[alexwennerberg]

Resolved in b3db9dc73f26630aa33239132fee206da804dd0e