Add description comment to POT file

[swissspidy]

As noted in #47 (comment):

I just noticed that the original makepot.php script has a description field for projects, e.g. Translation of the WordPress theme {name} {version} by {author}. However, that appears to be unused.

We might want to add this as a default comment to the POT file though as it can be useful when opening a POT file without context.