docs: write gateway.md (#283)

* doc: write gateway.md

* docs: address reviews (fix wording)

doc/src/contracts/gateway.md

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+# Gateway
+
+The name `gateway` used in this documentation refers to those entities which reside
+on axelar chain, which can also be called internal gateways. On the other hand we have
+external gateways, which are gateways deployed on external chains connected to Axelar. 
+
+The gateway contract is how messages enter the amplifier protocol. 
+Here are the steps taken throughout the lifecycle of
+a message:
+1. User sends a message to the external gateway. We call this incoming message.
+2. Incoming messages are sent to the gateway via `VerifyMessages`.
+3. The gateway calls `VerifyMessages` on the verifier, which submits the messages for verification (or just returns true if already verified).
+4. The messages are verified asynchronously, and the verification status is stored in the verifier.
+5. Once the messages are verified, `RouteMessages` is called at the gateway, which forwards the verified messages to the router.
+6. The router forwards each message to the gateway registered to the destination chain specified in the message.
+7. The prover retrieves the messages from the gateway, organizes them into a batch and submits the batch for signing.
+8. The relayer sends the signed batch to the external gateway.
+
+
+## Gateway graph
+```mermaid
+flowchart TD
+subgraph Axelar
+    Sg{"Source Gateway"}
+    Dg{"Destination Gateway"}
+    Rt{"Router"}
+end
+Rl{"Relayer"}
+Eg{"External Gateway"}
+
+Rl -- "Listen for Message:M1 Event" --> Eg
+Rl -- "M1" --> Sg
+Sg -- "M1" --> Rt
+Rt -- "M1" --> Dg
+```
+
+
+## Interface
+
+```Rust
+pub struct InstantiateMsg {
+    pub verifier_address: String,
+    pub router_address: String,
+}
+```
+
+As you can see, the gateway only needs to know the address of the two contracts it
+works with, which are voting verifier and router.