Renamed notes as markdown files.

This just makes the note files render more nicely for github.

Compare the markdown [Architectural Thoughts.md](https://github.com/Davorak/ComonadSheet/blob/ee23c3a80652e7fa798ed9ef05b885b7176624a7/Notes/Architectural%20Thoughts.md) and the old txt [Architectural Thoughts](https://github.com/kwf/ComonadSheet/blob/fc8fb61ff63f84404bed33ae65c3a331e64af334/Notes/Architectural%20Thoughts.txt)