Clarify need that X.509 certificate must be part of the IGTF to be able to use an API credential

[gwarf]

We recently had an user unable to use a non IGTF certificate to access the API using a credential added in the GOCDB, and this requirement is not mentioned on https://gocdb.github.io/api/

[gregcorbett]

Thanks for this: the requirement for an IGTF X.509 certificate is stated here: https://gocdb.github.io/api/read/, but I agree there should be a reference to the requirement at https://gocdb.github.io/api/.

Also, https://gocdb.github.io/api/read/#data-protection-levels and https://gocdb.github.io/api/write/#authenticationauthorisation should probably include a link to https://gocdb.github.io/api/#api-credential-mechanism.