Add CONTRIBUTING.md
Add CONTRIBUTING.md

Mainly it is similar to Bitcoin Contribution Guide


Former-commit-id: 5beac16ef15297e34d3e15c101789eaa0b32bdae
exonum/exonum,alekseysidorov/exonum,exonum/exonum,alekseysidorov/exonum,alekseysidorov/exonum,exonum/exonum,alekseysidorov/exonum,exonum/exonum