[doc] current rbf policy

doc/policy/README.md

@@ -1,10 +1,15 @@
 # Transaction Relay Policy
 
-Policy is a set of validation rules, in addition to consensus, enforced for unconfirmed
-transactions.
+**Policy** (Mempool or Transaction Relay Policy) is the node's set of validation rules, in addition
+to consensus, enforced for unconfirmed transactions before submitting them to the mempool. These
+rules are local to the node and configurable (e.g. `-minrelaytxfee`, `-limitancestorsize`,
+`-incrementalRelayFee`). Policy may include restrictions on the transaction itself, the transaction
+in relation to the current chain tip, and the transaction in relation to the node's mempool
+contents. Policy is *not* applied to transactions in blocks.
 
 This documentation is not an exhaustive list of all policy rules.
 
+- [Mempool Replacements](mempool-replacements.md)
 - [Packages](packages.md)