document how to use the helm chart with db2

[lmsurpre]

currently the readme says this:

The deployer can specify custom datasource named templates which will be used in the generation of the datasource.xml and bulkdata.xml files by overriding the datasourcesTemplate chart value. The default for this chart value is a datasources template for a Postgres database, but this helm chart also provides named templates for Db2, Db2 on Cloud, and Derby databases in the _datasourcesXml.tpl file.

and the values doc for datasourcesTemplate says:

Template containing the datasources.xml content

in neither place, does it actually tell you the name of our built-in db2 template. too much work for users, we should fix that