[packages/doc] define and document package rules

Central place for putting package-related info. This document or parts
of it can also be easily ported to other places if deemed appropriate.

doc/policy/README.md

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+# Transaction Relay Policy
+
+Policy is a set of validation rules, in addition to consensus, enforced for unconfirmed
+transactions.
+
+This documentation is not an exhaustive list of all policy rules.
+
+- [Packages](packages.md)
+
+