Update README
Introduced a tool, `passenger-config --detect-apache2`, which autodetects all Apache installations on the system along with their parameters.

That is, which apachectl command to run, which log file to read, which config file to edit.
The tool advises users about how to use that specific Apache installation.
Useful if the user has multiple Apache installations but don't know about
it, or when the user doesn't know how to work with multiple Apache installations.