Document phpdbg / pcov / xdebug uses briefly

[stronk7]

Over different issues (#137, moodlehq/moodle-docker@46cccbe, #17 (comment)...) there are spread comments and instructions about how to use phpdbg / pcov / xdebug with the PHP images.

This issue is about add some information about them in the README, so people doesn't have to look everywhere to know more about those 3 integrations (phpdbg / pcov for code coverage and xdebug for that and more).

This closes #17.

[stronk7]

Just to note that phpdbg is far from ideal for PHP >= 8.0. It crashes all the time. More yet, I've read somewhere (cannot find the link) that it may be removed from php distros soon.

So, surely, when documenting that, we should, somehow, clarify that pcov > xdebug > (nonono) phpdbg are the preference. That, pretty much, matches what we do in moodle-plugin-ci (and surely other tools).

Ciao :-)