Install and configuration documentation

This is a great module - but there are a lot of important details to getting it work that at the moment exist only buried deep in old issues or through trial and error.

It'd be great if there was a documentation page that sets out how to get this up and running and some of the issues to watch out for, including:

• A simple explanation that Ultimate Cron uses background processes that are either url-based HTTP requests or PHP CLI commands, depending on what called the cron (and for now, a note that you need -dev for the latter)
• An example drush cron command that works, e.g. like /path/to/drush -r /path/to/drupal cron-run all --cli --check-rule > /dev/null 2>&1 with friendly annotations noting things like, that many cron environments will need the full path to drush unless drush has been configured to be in the cron user's path, explaining '--check-rule', explaining output dumping to dev/null, etc
• A plug for the optional Background Process Apache Status module bundled with Background Process, with a link to a Background Process documentation page that explains that you need the Apache Server Status apache module enabled, and that shows how you can use server status info in debugging (e.g. like, or linking to, #1443264)
• An explanation of what it means when a task is stuck, with some of the common causes and fixes from #1443264, in particular:
• A note that builders should check that their Apache max client setting is equal or higher to the Ultimate Cron max connections setting, and that the latter needs to be lowered if it's higher
• A note suggesting using the drupal settings code to protect against memory exhaustion causing silent failure
• A clear note about where the first place to look for Ultimate Cron errors should be