Elaborate on OSSEC alert instructions

Addresses #788 
Need to make edits to install.md to account for new document
Maybe add a note saying that we don't recommend enabling 2-step verification if you're using a new account that is dedicated to relaying the alert emails. It's unnecessary and adds an extra layer that could potentially cause problems. If you pick a strong random passphrase (generated and stored in KeePassX, using our template) the relay will be as or potentially even more secure than if you use a Google App Password.