[mastodoncomments] Installation instructions fail if `files/` doesn't exist

[mtorpey]

The line

You need to copy the folder $BLOG_ROOT/plugins/mastodoncomments/files/assets to $BLOG_ROOT/files/assets

is followed by

cp -a plugins/mastodoncomments/files/assets files/

which looks correct, but gets confused if files/ doesn't exist yet (as in my case). In that case we need to create files/ first. Something like:

[ -d "files" ] || mkdir files
cp -a plugins/mastodoncomments/files/assets files/

should do it, or else text telling the user to make sure files/ exists.