Add information about standard comment markers

[dragonee]

I use TODO and XXX regularly (often together), and sporadically put FIXME in the code.

http://www.catb.org/jargon/html/X/XXX.html
http://www.catb.org/jargon/html/F/FIXME.html
TODO is self-explanatory.

The list is open, the bottom line is to use the same opinionated nomenclature.

Added value that most text editors and static analysis tools recognise them.