added some docs about running LH headless.
added some docs about running LH headless.

tommycli/lighthouse,tkadlec/lighthouse,ev1stensberg/lighthouse,GoogleChrome/lighthouse,mixed/lighthouse,arturmiz/lighthouse,mixed/lighthouse,umaar/lighthouse,cedricbellet/lighthouse,ev1stensberg/lighthouse,GoogleChrome/lighthouse,umaar/lighthouse,umaar/lighthouse,cedricbellet/lighthouse,tommycli/lighthouse,deepanjanroy/lighthouse,tommycli/lighthouse,cedricbellet/lighthouse,arturmiz/lighthouse,ev1stensberg/lighthouse,GoogleChrome/lighthouse,cedricbellet/lighthouse,tommycli/lighthouse,wardpeet/lighthouse,wardpeet/lighthouse,cedricbellet/lighthouse,GoogleChrome/lighthouse,mixed/lighthouse,wardpeet/lighthouse,umaar/lighthouse,ev1stensberg/lighthouse,arturmiz/lighthouse,arturmiz/lighthouse,GoogleChrome/lighthouse,mixed/lighthouse,deepanjanroy/lighthouse,wardpeet/lighthouse,wardpeet/lighthouse,tommycli/lighthouse,arturmiz/lighthouse,deepanjanroy/lighthouse,tkadlec/lighthouse,deepanjanroy/lighthouse,tkadlec/lighthouse,tkadlec/lighthouse,deepanjanroy/lighthouse