How do I add a comment in Vim's configuration files, like .vimrc?
Put a double quote to the left of the text you want to comment:
" this is how a comment looks like in ~/.vimrc
(You don't need a closing quote.)
This comment syntax is documented at :help comment, part of docs on Ex command-lines, which begins:
'"' at the start of a line causes the whole line to be ignored. '"'
after a command causes the rest of the line to be ignored. This can be used
to add comments. ...
(Note, per :help vimrc, that "Each line in a vimrc file is executed as an Ex command line.")