Add iamrole notice.

What does this MR do?

Adds a notice to the docs to let user's know how to use iamrole's without secret/access keys.

Why was this MR needed?

Some clients thought that they were forced to use secret/access keys. This caused confusion and upsetness.

Are there points in the code the reviewer needs to double check?

No.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?