move mentor walkthrough notes into file on master branch

See https://github.com/foundersandcoders/master-reference/issues/456
> generally against having things on branches. Means that as the workshop is iterated, you have to keep merging master into the notes/solutions branch. I like keeping that kind of stuff in a dotfile or dotfolder in the root, like how I did for foundersandcoders/ws-software-design-js