Excessive scope for network_os README note

[tremble]

The following warning is currently added to the README:

For collections that support Ansible 2.9, please ensure you update your `network_os` to use the 
fully qualified collection name (for example, `cisco.ios.ios`). 

While ansible-network is nominally scoped to network modules, many of the release related tools have ended up with a much broader scope and are just maintained by the network engineering team. As far as I can tell this warning only applies to a handful of collections, however it's currently being spammed into any that use collection_prep. Please make this warning optional and provide a mechanism to disable it which can be stored in the repo.