Consolidate MySQL connector doc

[crandmck]

I would like to move the information in https://github.com/strongloop/loopback.io/blob/gh-pages/pages/en/lb2/MySQL-connector.md into https://github.com/strongloop/loopback-connector-mysql/blob/master/README.md so we can have a single source for doc for this connector.

The Basic use section seems questionable because we typically document for use with LoopBack and LB tools, so the juggler dependency is assumed, and the data source is added by using the data source generator (e.g. apic/slc loopback:datasource). I believe this is covered by Creating a MySQL data source.

Once we move the content into the connector README, I will copy the content of https://github.com/strongloop/loopback.io/blob/gh-pages/pages/en/lb2/mysql-README.md into https://github.com/strongloop/loopback.io/blob/gh-pages/pages/en/lb2/MySQL-connector.md, (and delete mysql-README.md from the loopback.io repo.

Then our Jenkins pipeline will ensure that the content from the README is automatically incorporated into the docs.

cc @Amir-61