Create a public and generic "Private Key Address Specification" doc

[pccourt]

If we are going to use distributed ledger / block chain based PrivateKeyAddress fields within the OB taxonomy, we also need to reference (or have) robust documentation on what this is and how to use it. Lumian have a currently private doc that defines their "Private Key Address Specification", the OB taxonomy needs to reference a public version of something similar to this or to ideally create a more generalized and public OB specific version of a "Private Key Address Specification".