Updated the robots.txt
Updated the robots.txt

This file updates robots.txt, and I modified it to block some of the common web spiders which crawl the site but don't give us any benefit.
pellaea/mushroom-observer,JoeCohen/mushroom-observer,pellaea/mushroom-observer,MushroomObserver/mushroom-observer,JoeCohen/mushroom-observer,JoeCohen/mushroom-observer,JoeCohen/mushroom-observer,MushroomObserver/mushroom-observer,JoeCohen/mushroom-observer,pellaea/mushroom-observer,MushroomObserver/mushroom-observer,JoeCohen/mushroom-observer,pellaea/mushroom-observer,pellaea/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,JoeCohen/mushroom-observer,pellaea/mushroom-observer,MushroomObserver/mushroom-observer,pellaea/mushroom-observer,MushroomObserver/mushroom-observer