Add information about "system check" load chunk (now also in the regular atari target).
Add information about problems when upgrading (MAINHDR).
Add information about to get rid of "system check" load chunk.
Move "Technical details" in front of "Other hints".
Merge pull request #52 from groessler/gh-pages_to_pull

Atari documentation update
Update 'Reserving a memory area inside a program' chapter to current reality.
Some other small changes.
Merge pull request #53 from groessler/gh-pages_to_pull

More Atari documentation updates