add coordinator doc (axelarnetwork#435)

doc/src/SUMMARY.md

@@ -11,6 +11,7 @@
 - [Voting Verifier](contracts/voting_verifier.md)
 - [Multisig Prover](contracts/multisig_prover.md)
 - [Rewards](contracts/rewards.md)
+- [Coordinator](contracts/coordinator.md)
 
 # Contributing
 

doc/src/contracts/coordinator.md

@@ -0,0 +1,19 @@
+# Coordinator
+
+Some contracts, like the multisig provers, are deployed per chain and unknown to one another. The coordinator contract keeps track of these chain-dependent provers and coordinates any interaction that requires knowledge of all of them. For example, the ability for a verifier to unbond their stake requires to check that it's not part of any active verifier set on any chain.
+
+```mermaid
+flowchart TD
+
+Co{"Coordinator"}
+Go{"Governance"}
+PrA{"Prover A"}
+PrB{"Prover B"}
+PrC{"Prover C"}
+
+
+Go -- "Register Prover contract" --> Co
+PrA  -- "Set active verifiers" --> Co
+PrB  -- "Set active verifiers" --> Co
+PrC  -- "Set active verifiers" --> Co
+```