Document MySQL credential environment variable support

[ldcasillas-progreso]

The documentation currently does not say very clearly what are the options to provide host, username and password for a MySQL source. The code, however, shows that some standard MySQL environment variables are supported:

• pgloader/src/parsers/command-mysql.lisp

  Line 101 in 8fb542b

  ;; Default to environment variables as described in

This should be documented.

[dimitri]

Would you consider contributing a documentation patch to that end?