add @aaronj1335's solution for doublets

s/o to @mwfogleman for [the idea to use tree-seq][the setup tho].

[the setup tho]: https://mwfogleman.github.io/posts/20-12-2014-flatcat.html
Update README.md
draws aren't really possible
Merge pull request #10 from aaronj1335/doublify

add @aaronj1335's solution for doublets
Changing first kata to alphabet cipher
Add some hints
Move hints
Minor text update (doublet->alphabet cipher)
Merge pull request #11 from paulhenrich/readme

Minor text update (doublet->alphabet cipher)