Document $PORT and $PS (#562)
Document $PORT and $PS (#562)

* Document $PORT and $PS

Update the ronn documentation to explain that $PORT and $PS exist and
that $PORT starts as the base port number and is incremented on each
line.

* Explain in more detail how $PORT changes

My prior explanation did not take `-c` into account.

ddollar/foreman,ddollar/foreman,ddollar/foreman