Mention of `/gh_event` endpoint missing in README

[stefanluptak]

It took me some time to realize, that I have to point the GitHub webhook to the /gh_event route of the exporter. It would be useful to have it mentioned in the README.
Also, there's probably a mistake in the description of the web.gh-webhook-path option in the main.go file at the line 29.

[cpanato]

thanks @stefanluptak, are you willing to open a PR to fix those things? otherwise I will do that when I get a free cycle

[stefanluptak]

OK, I'll try. :-) I thought about that, but then I realized, that maybe you will know better how to put the word in the README. But I'll try my best. :-)