Update README with ssl notes

[arndt-s]

Please update the README file and add a section where ssl is documented:

I've had a problem that my configuration in config/cms.php was overwritten by CMS_LINK_POLICY env of the docker image. Same goes for APP_URL etc.

[petehalverson]

Hi @arndt-s!

Agreed. I think we should add some helpful HTTPS examples.

By default, APP_ENV=docker when a container is started. The variables defined in config/docker/cms.php will override those set in config/cms.php unless otherwise overridden.

[programster]

An example in the README would be very useful.

It took me a while to figure out all the things I had to do. If it helps, I posted my solution (using docker with jwilder reverse proxy) here: https://blog.programster.org/deploy-october-cms-with-docker#adding-ssl