Link to freshly released coalaip spec

[TimDaub]

Everywhere.

One example:

• https://github.com/bigchaindb/coalaip-http-api/blob/master/README.md#register-a-right-allowed-by-another-right-or-copyright

Note that the attributes for the right may be much more diverse; see the COALA IP models definition (not yet publicly available yet).