cep: add README files

This patch adds a new README file that breifly discusses the CEP concept
and provides a guideline on how to write new CEPs. It also adds a brienf
entry to the top-level README file of the whole tree.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/cep/ by tarmac [r=sylvain-pineau][bug=][author=zkrynicki]"