Add info to README describing how to start using a custom port (#75)
Add info to README describing how to start using a custom port (#75)

* Add info to README describing how to start using a custom port

- Added a subsection describing how to start RSparkling on a custom
port.
Driveby fix: Made notes bolded for consistency rather than headings.

* additional edits after viewing in browser

* updates from review

h2oai/rsparkling,h2oai/rsparkling