Update documentation to explain how Hugo modules work

[mattstratton]

With the change to using the theme as a module, in order to run it locally you need a few things

1. you need Go installed locally and of a not super old version (details needed)
2. you may need to do hugo mod get -u in the repo directory the first time you clone it, etc to get the modules installed
3. we probably need to do some updates to the dockercontainer etc