Update macro list

Put in correct number of possible $USERx$ variables.
Merge pull request #7 from awiddersheim/patch-1

Update macro list
What is Naemon, Naemon Core, Naemon Suite and Nagios Core Core?
about.md: Minor language and spelling corrections
Merge pull request #8 from catharsis/patch-2

about.md: Minor language and spelling corrections
Update about.md

fixed typo