MSSQL connector has a README

[izgeri]

Until there is official documentation on our docs site, there should be a README in the MSSQL connector dir that describes how to config and use the connector

From the AC in #968:

• Users can use Secretless with the MSSQL connector to deploy applications to Kubernetes or OpenShift that require an MSSQL Server 2017 v14.x database that the apps authenticate to using using SQL Server authentication
• Users can create applications that connect to MSSQL instance with no credential knowledge
• New connections to MSSQL always use fresh credentials from the configured credential provider(s)
• MSSQL connector config is consistent with config for other Secretless database connectors (eg username, password, host, and (optional) port), and the standard default for port (eg 1433) is enforced