How to comment in Vim's config files: ".vimrc"?

A double quote to the left of the text you want to comment.

Example: " this is how a comment looks like in ~/.vimrc

To be clear: Do not include a closing double quote. (See comment from Arslan Ali below.)

You can add comments in Vim's configuration file by either:

" brief description of the command    

or:

"" commented command

Taken from here