First proposal how to add .well-known URI location to xRegistry

As discussed in the meeting series, here is a first draft how we could incorporate the .well-known URI concept into the xRegistry.

This proposal only contains the minimum: Pointing to the xRegistry API location, which protocol version it is supporting and its capabilities.

Related: #120 Related: #106

Proposed Changes

add .well-known/xregistry.json as the entry point to an xRegistry server
describe there where to find the xRegistry APIs (in the future, we may have v1 and v2) and their supported capabilities
add top-level information that are free to give out openly (non-sensitive)
adds modelurl to point to xRegistry metadata (or should this be protocol version dependent, too?)

Release Note

TODO: Will write this after we reviewed this.