simplify install doc since forwarder is go-gettable

I totally understand why it might make sense to keep the original phrasing, as this way requires a pre-existing `$GOPATH`. For potential developers who need `$GOPATH` set up anyway, that's great. For people just trying to install logstash-forwarder, that may be an unsafe assumption.
That advice only holds if you run go get from $GOPATH :( - the most technically correct note would be that LSF lives at $GOPATH/bin/logstash-forwarder