Update README.md
Merge pull request #54 from pvieytes/patch-1

Update README.md
use ROLE instead of USER [pgsql]

this supports e.g. the REPLICATION role later
SELECT 1 is sufficient for user check [pgsql]
overwrite password attribute for user provider [pgsql]
add option attributes, default to LOGIN [pgsql]
update user provider to support options [pgsql]
use "on" attribute for grant statement [pgsql]
update README to reflect pgsql provider changes