Clarify documentation

[ctgraham]

Add additional context to the README to explain the use cases and requirements behind the module, with details on the required configuration fields for the Action form.

Also add copyright, licenses and maintainer references to the README.

The default value of "finding_aid.xsl" for the transform file path is too specific, and should be removed.

xslt_derivative/src/Plugin/Action/XsltDerivative.php

Line 137 in eff016e

'transform_path' => 'finding_aid.xsl',