Document Gitian (reproducible) build procedure for Omni Core

[msgilligan]

There are no (Gitian) build instructions in https://github.com/OmniLayer/omnicore/tree/develop/src/omnicore/doc and the Bitcoin Core Gitian build instructions in https://github.com/OmniLayer/omnicore/blob/develop/doc/gitian-building.md provide a link that is no longer valid.

I propose creating a document omni-gitian-build.md in src/omnicore/doc and having that be our official instructions until we move to GUIX (pronounced “geeks”) as Bitcoin Core has already done — and then create an omni-guix-build.md document. Those documents can possibly link to additional or background instructions elsewhere and can be updated for each release.

[msgilligan]

These are the archived upstream instructions:
https://github.com/bitcoin-core/docs/blob/38befdf71ba21e69a598e974f7c06a9cdb5b0dfd/gitian-building.md