[#1] [E-99] Adding description for problems from 21st to 25th.
[#1] [E-99] Working solution for P21.
[#1] [E-99] Working solution for P22.
[#1] [E-99] Working solution for P23.
[#1] [E-99] Working solution for P24 and P25.
Merge pull request #4 from afronski/erlang-learning

Pull request for 99 problems in Erlang implementation