Slight rewording in section 2.1.1.

change 'read' to 'appropriately handle' for fits parsers. The point
of versioning is not that we forevermore make the format bulletproof
for readers, but rather that readers can appropriately handle conditions
occuring in the file (e.g. new structures they dont know about can
throw exceptions, be skipped over, warnings emitted, etc) which doesnt
really mean they can read the file.
Mark text with note to find data model definition.

Some have expressed doubts about using Wikipedia as source
for definition of 'data model'. Doubtless there are other
CompSci papers we might reference for this definition?
Clean up maxim refs, remove red comment

We dont need to be exhaustive about referencing the maxim. If
one reference has the maxim a little different, I think its ok
to drop it. 2 papers is enough to get the point across.