Readme build instructions

[dean-krueger]

Previous versions of the README left users (me when I was trying to do this for the first time) high and dry after telling them to run make docker-gh-preview. It was not at all clear that to actually view the changes made to the website it needed to be served and visited at localhost. This version of the README gives users (especially new ones) more detailed instructions on exactly what they need to do after they make changes to the website so that they can check their work.

Closes #397

[dean-krueger]

should be python3 -m http.server <port number>. Oops. I'll change that momentarily.

[gonuke]

A comment about port-forwarding and when it's necessary.

Also, does it make sense to describe how to serve it from inside a docker container and access it? Or do we assume/instruct people to bind mount everything and then launch the server from outside the docker container?

[dean-krueger]

should be python3 -m http.server <port number>. Oops. I'll change that momentarily.

Fixed

[gonuke]

LGTM - thanks @dean-krueger

[gonuke]

Oh - needs a rebase or similar

[dean-krueger]

I hit the "Update Branch --> Rebase" button here on GitHub and it looks like it did a bunch of stuff automatically for me. Not entirely sure if that was good or not, so I'm going to take another quick look. It might be worth the same from @gonuke before we go and hit any buttons (I'll probably not hit that button again).

[dean-krueger]

It looks like this should be fine? I guess worst case scenario it messes up the website and then I spent the weekend fixing it.

[gonuke]

Thanks @dean-krueger