Caesar cipher exercise

[bridgethillyer]

@msszczep made a very nice exercise that could be included in the curriculum. Can we put this in community-docs for now? Where is the right place for it?

https://gist.github.com/msszczep/b20b847a7e8ddc6fbad4

[yokolet]

I created "exercises" section and added the contents of the gist.
http://clojurebridge.github.io/community-docs/docs/exercises/caesar-cipher/
How do you think? @bridgethillyer, @msszczep

[bridgethillyer]

Looks great @yokolet. Thank you!!!