Document Keybase Error Adding IAM Users

[osterman]

what

aws_iam_user_login_profile.default: Error retrieving Public Key for keybase:testuser: unable to fetch keys for user(s) "testuser" from keybase

You can verify this error, but running something like:

curl https://keybase.io/testuser/key.asc

Which, if not setup correctly will look like this:

SELF-SIGNED PUBLIC KEY NOT FOUND

To fix this, the user should make sure they've added their PGP key to Keybase. They should see something like this: