“Instapaper turns web content – articles, stories, posts, videos, and even long emails – into a great reading experience.” If you have an Instapaper account, you may wish to send your links there at the same time as you send them to Pinboard. You do this with the --with-instapaper option, but you’ll need to provide credentials.
You can give your username & password with --username and --password, but see also Providing Your Credentials.