Add add-gpg-user --trusted documentation to the READMEs

[spk]

Hi completely forgot that we need to trust the key when adding a collaborator to the repository having error like:

gpg: XXX: There is no assurance this key belongs to the named user
gpg: [stdin]: encryption failed: Unusable public key
git-crypt: GPG error: Failed to encrypt

ref https://github.com/AGWA/git-crypt/issues/23

Cheers, Laurent