Document migration from SHA1 for existing keys

I encounter the error

```
▶ ERROR key generation error: no valid primary key self-signature or key(s) have expired (Signature failure in packet 1: rejecting insecure hash SHA1 (98a50086f826a705)) (error 905)
```

when attempting to upload my public key to Keybase. This prevents me from using Keybase at all.

I see that there is a workaround at https://github.com/keybase/client/issues/22458#issuecomment-584179988 with the strong disclaimer

> I have a limited knowledge of what might break if you do this and other people already have your public key. Also if my real keys where at stake here, I would backup my secret keys beforehand.

As such, I do not feel comfortable following the procedure. Could you (the Keybase maintainers) please publish an officially endorsed and verified-safe procedure for working around this error?

The issue names no repository files or tests; begin by reviewing the linked workaround and the reported SHA1 key-generation error. Done means publishing an officially endorsed, verified-safe procedure for migrating existing keys, including guidance that addresses backups and keys already shared.

Written by the indexing model from the issue text.