Add a section in the README.md about requiring and auto-attach activation key.

[bschonec]

In order for this module to work properly, an auto-activate activation key needs to be created and used.

I spent an hour or so using an activation key that didn't have the auto-attach flag set until I realized that the auto-attach needs to be enabled.

I suggest putting in some verbiage in the README.md reminding the users of this fact.

[kallies]

If the module does not work without auto-attach, changes to the module are needed (to be confirmed). Background is the way Red Hat handles (newly) created manifests. Simple Content Access does not require auto-attach and this feature is therefore disabled.

[bschonec]

Please be aware that subscription-manager does not support auto-attach feature when using an activation key.

[root@example.com security]# subscription-manager register --activationkey=my_activation_key --org=my_org--auto-attach
Error: Activation keys cannot be used with --auto-attach.