message: Clarify the nature of the SDK.
diff: # Indy SDK
  +This is the official SDK for [Hyperledger Indy](https://www.hyperledger.org/projects),
  +which provides a distributed-ledger-based foundation for [self-sovereign identity](https://sovrin.org).
  +The major artifact of the SDK is a c-callable
  +library; there are also convenience wrappers for various programming languages.
  +
  ## Building of Indy SDK
  * [Ubuntu based distro (Ubuntu 16.04)](doc/ubuntu-build.md)
  