added text about case-insensitive file systems and dots in names

- about file names: this is from my experience - i've had some nasty problems with capitalization of file names on an svn with linux/win clients.
- about dots-in-names: i think this is a nice addition to demonstrate the advantages of using underscores vs. dots.

i hope this makes sense. ;)

I assign the copyright of this contribution to Hadley Wickham.