deploy-sourcegraph: Add documentation about using LetsEncrypt

Created by: ggilmore

Related: https://github.com/sourcegraph/sourcegraph/issues/1609

We should add a section in https://github.com/sourcegraph/deploy-sourcegraph/blob/master/docs/configure.md#configure-tlsssl that shows you how to use lets encrypt instead of needing to create your own cert.

The docs will mostly follow https://github.com/sourcegraph/about/blob/master/docs/server/tutorials/tlsssl.md#lets-encrypt, but we should note that both port 80 and port 443 need to exposed on sourcegraph-frontend for this to work.

cc @slimsag