Use contributor license agreement & GitHub integration

[slowli]

Contributor license agreements are used to more precisely define contribution terms for developers. Some examples:

• Apache Foundation
• Eclipse
• JS Foundation
• GitHub

Correspondingly, they could be useful to all public repos in the exonum org.

CLA assistant can be used to setup CLA sign-off for contributors. It comes with a GitHub integration, among other things. Note that there is an option in the assistant to link an entire org to a CLA, which seems to be the preferable way in our case.

Steps forward:

• Test on a test repo, e.g., on @djsatok's example-doc, if that doesn't overburden Alex; alternatively, I can create a test repo myself.
• Create & finalize CLA text. Developer Certificate of Origin (DCO) or Apache's ICLA or SAP's ICLA (comes with the CLA assistant) seem like good starting points
• Expand to this repo and other repos in the exonum org. I'm not sure it's possible while the repos are still private, but we can try.