Add docs/sequences.md with deposit, withdraw, and exit sequences

docs/bridge/sequences.md

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+## Deposit
+
+Bridge tokens from rootchain to childchain via deposit.
+
+```mermaid
+sequenceDiagram
+	User->>Edge: deposit
+	Edge->>RootERC20.sol: approve(RootERC20Predicate)
+	Edge->>RootERC20Predicate.sol: deposit()
+	RootERC20Predicate.sol->>RootERC20Predicate.sol: mapToken()
+	RootERC20Predicate.sol->>StateSender.sol: syncState(MAP_TOKEN_SIG)
+	Edge->>User: ok
+	RootERC20Predicate.sol-->>Edge: TokenMapped Event
+	StateSender.sol-->>Edge: StateSynced Event
+	Edge->>StateReceiver.sol:commit()
+	StateReceiver.sol-->>Edge: NewCommitment Event
+	Edge->>StateReceiver.sol:execute()
+	StateReceiver.sol->>ChildERC20Predicate.sol:onStateReceive()
+	ChildERC20Predicate.sol->>ChildERC20.sol: mint()
+	StateReceiver.sol-->>Edge:StateSyncResult Event
+```
+
+## Withdraw
+
+Move tokens from childchain to rootchain via withdrawal.
+
+```mermaid
+sequenceDiagram
+	User->>Edge: withdraw
+	Edge->>ChildERC20Predicate.sol: withdrawTo()
+	ChildERC20Predicate.sol->>ChildERC20: burn()
+	ChildERC20Predicate.sol->>L2StateSender.sol: syncState(rootToken, WITHDRAW_SIG), recv=RootERC20Predicate
+	Edge->>User: tx hash
+	User->>Edge: get tx receipt
+	Edge->>User: exit event id
+	ChildERC20Predicate.sol-->>Edge: L2ERC20Withdraw Event
+	L2StateSender.sol-->>Edge: StateSynced Event
+	Edge->>Edge: Seal block
+	Edge->>CheckpointManager.sol: submit()
+```
+## Exit
+
+Finalize withdrawal of tokens from childchain to rootchain.
+
+```mermaid
+sequenceDiagram
+	User->>Edge: exit, event id:X
+	Edge->>Edge: bridge_generateExitProof()
+	Edge->>CheckpointManager.sol: getCheckpointBlock()
+	CheckpointManager.sol->>Edge: blockNum
+	Edge->>Edge: getExitEventsForProof(epochNum, blockNum)
+	Edge->>Edge: createExitTree(exitEvents)
+	Edge->>Edge: generateProof()
+	Edge->>ExitHelper.sol: exit()
+	ExitHelper.sol->>CheckpointManager.sol: getEventMembershipByBlockNumber()
+	ExitHelper.sol->>RootERC20Predicate.sol:onL2StateReceive()
+	RootERC20Predicate.sol->>RootERC20: transfer()
+	Edge->>User: ok
+	RootERC20Predicate.sol-->>Edge: ERC20Withdraw Event
+	ExitHelper.sol-->>Edge: ExitProcessed Event
+```
+