Consolidate PostgreSQL connector doc

[crandmck]

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

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/postgresql-README.md into https://github.com/strongloop/loopback.io/blob/gh-pages/pages/en/lb2/PostgreSQL-connector.md (then delete postgresql-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