FR - Add documentation on how to use existing keys

[akshetpandey]

It would be amazing if you can also tell us how to use existing gpg keys that we have and how to upload that to keybase. Also, how to use an existing keybase account (with key).

[rogeruiz]

@akshetpandey This worked for me for an existing Keybase account. I can add a PR if that's helpful.

# step 1, list your Keybase ID
$ keybase pgp list
Keybase Key ID:  MY_KEYBASE_ID
PGP Fingerprint: MY FIN GER PRI NT 
PGP Identities:
    <hello@example.com>
# then, export your secret key and type your passphrase into the keybase prompt
$ keybase pgp export -s | gpg --import --allow-secret-key-import
# follow the next two instructions from the README.md
# https://github.com/pstadler/keybase-gpg-github#set-up-git-to-sign-all-commits
# https://github.com/pstadler/keybase-gpg-github#add-public-gpg-key-to-github
# for the second step, use the MY_KEYBASE_ID for the `--query` / `-q` part

If you're just looking for existing gpg keys to put into Keybase. You just need to export from gpg and import using Keybase. That's in the Keybase docs. Try the command keybase pgp help import to get started with importing gpg generated keys.