Add bookmarklet link

"Use mirrorrr anywhere, anytime with this bookmarklet".
When clicked while on ```https://mirrorrr.appspot.com```, it tells you to drag itself to your browsers' bookmarks bar. When clicked anywhere else, it feeds the current URL into ```mirrorrr``` and loads it up.
Can you make this use the hostname of the current request? Like window.location.hostname? A lot of people fork this repo and host it on their own *.appspot.com domains.