Create RFC for SDK versioning

[mversic]

We need to standardize process of versioning SDKs:

• SDKs should follow semantic versioning
• every bump of Iroha_core should result in bump of major version of an SDK
• SDK can bump it's major version without bumping iroha_core dependency, but
• every bump of major version of an SDK must contain in the documentation which Iroha version it corresponds to