Add DMP to the glossary

roadmap/implementers-guide/src/glossary.md

@@ -7,6 +7,7 @@ Here you can find definitions of a bunch of jargon, usually specific to the Polk
 - Backed Candidate: A Backable Candidate noted in a relay-chain block
 - Backing: A set of statements proving that a Parachain Candidate is backable.
 - Collator: A node who generates Proofs-of-Validity (PoV) for blocks of a specific parachain.
+- DMP: Downward Message Passing. Message passing from the relay-chain to a parachain.
 - Extrinsic: An element of a relay-chain block which triggers a specific entry-point of a runtime module with given arguments.
 - GRANDPA: (Ghost-based Recursive ANcestor Deriving Prefix Agreement). The algorithm validators use to guarantee finality of the Relay Chain.
 - HRMP: (Horizontally Relay-routed Message Passing). A mechanism for message passing between parachains (hence horizontal) that leverages the relay-chain storage. Predates XCMP.