Yes, you may put comments in there. They however must start at the beginning of a line.
cf. http://git-scm.com/book/en/Git-Basics-Recording-Changes-to-the-Repository#Ignoring-Files
The rules for the patterns you can put in the .gitignore file are as follows:
– Blank lines or lines starting with # are ignored.
[…]
The comment character is #, example:
# no .a files
*.a