Docs: Document Conformance Deprecation Policy

[danehans]

Several conformant gateway implementations have not submitted a report for the v1.0.0 release. The project should define a simple conformance deprecation policy that removes an implementation from the implementations list when the implementation has not submitted a report for an agreed-upon amount of time, i.e. current release -2 minor releases.

[danehans]

xref: #1651

[danehans]

With a minimal set of v1.0.0 conformant implementations and v1.1.0 around the corner, I think it's important that this policy is documented.

cc: @nirrozenbaum @kfswain @robscott

[nirrozenbaum]

+1