docs(rollup): improve readme (#917)

Co-authored-by: HAOYUatHZ <haoyu@protonmail.com>

rollup/README.md

@@ -1,15 +1,23 @@
 # Rollup
 
-This repo contains the Scroll rollup.
+This directory contains the three essential rollup services for the Scroll chain:
+- Event Watcher (<a href="./cmd/event_watcher/">event_watcher</a>): watches the events emitted from the L1 and L2 contracts and updates the event database.
+- Gas Oracle (<a href="./cmd/gas_oracle/">gas_oracle</a>): monitors the L1 and L2 gas price and sends transactions to update the gas price oracle contracts on L1 and L2.
+- Rollup Relayer (<a href="./cmd/rollup_relayer/">rollup_relayer</a>): consists of three components: chunk and batch proposer and a relayer.
+    - The chunk and batch proposer proposes new chunks and batches that sends Commit Transactions for data availability and Finalize Transactions for proof verification and state finalization.
 
 ## Dependency
 
-+ install `abigen`
+1. `abigen`
 
 ``` bash
 go install -v github.com/scroll-tech/go-ethereum/cmd/abigen
 ```
 
+2. `solc`
+
+See https://docs.soliditylang.org/en/latest/installing-solidity.html
+
 ## Build
 
 ```bash