Mention `enable: true` when passing object to LR

I've realized that setting `filter` property in livereload object is not enough to activate the livereload. The object also needs to set `enable: true` for LR to kick in. Updating readme and adding one more FAQ to make it clear.
Merge pull request #63 from demisx/patch-1

Mention `enable: true` when passing object to LR