Document creating specific managers in the README.md

This is related to https://github.com/Azure/azure-libraries-for-net/issues/200#issuecomment-371219101 to help users understand that they can go through specific managers rather than the "rollup" manager when using our libraries.

I took a look at our samples an some of the tests and most of them use the "rollup" Azure manager.

Start with README.md and the referenced samples and tests to see how the libraries currently create managers. Use the linked issue comment for context, then document how users can create specific managers instead of relying on the rollup manager. Done means the README clearly explains the specific-manager approach.

Written by the indexing model from the issue text.