Comments in .gitignore?

Can you write comments in a .gitignore file?

If so, should the line be preceded with a # or some other indicator?

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

Do git help gitignore

You will get the help page with following line:

A line starting with # serves as a comment.