Update theme updating strategy and README documentation

[manuelvsousa]

in progress

git subtree pull --prefix themes/hugo-book https://github.com/alex-shpak/hugo-book.git v8 --squash
git subtree add --prefix themes/hugo-book https://github.com/alex-shpak/hugo-book.git v8 --squash

[terjanq]

I also did changes to the layout: #65

So just to make sure we will not break anything with adding a submodule.

[manuelvsousa]

Oh, I see. One of us will have to merge since I changed the theme directory to the original theme name. This is not a priority and I think your PR is more important. If it comes to that, I can do the process again after we merge your hint2 PR. In the meanwhile I can work on some documentation here temporarily.

[manuelvsousa]

#71 is the new PR for this