Add LetsEncrypt Instruction in README

[NoobTW]

Looks like here's a way to enable SSL cert. I think it's good to adding it to README.

nginx-proxy-manager-assistant/app/ContainerEvents.js

Lines 43 to 47 in 86b6ad5

	// Generate cert?
	let generateCert = data.Config.Env.find(item => {
	let kv = item.split('=');
	return (kv[0] == "GEN_CERT" && kv[1] == "true")
	});

[NoobTW]

Well, it turns out that the image on Docker Hub is not up-to-date with current repo. I'll have to clone the repo and build the image on my own to enable the GEN_CERT stuff.