feat: readme.md for data-fetcher service

packages/data-fetcher/README.md

@@ -1 +1,52 @@
 # zkSync Era Block Explorer Data Fetcher
+## Overview
+
+`zkSync Era Block Explorer Data Fetcher` service exposes and implements an HTTP endpoint to retrieve all data for a certain block / range of blocks from the blockchain. This endpoint is called by the [Block Explorer Worker](/packages/worker) service.
+
+## Installation
+
+```bash
+$ npm install
+```
+
+## Setting up env variables
+
+- Create `.env` file in the `data-fetcher` package folder and copy paste `.env.example` content in there.
+```
+cp .env.example .env
+```
+- In order to tell the service where to get the blockchain data from set the value of the `BLOCKCHAIN_RPC_URL` env var to your blockchain RPC API URL. For zkSync Era testnet it can be set to `https://zksync2-testnet.zksync.dev`. For zkSync Era mainnet - `https://zksync2-mainnet.zksync.io`.
+
+## Running the app
+
+```bash
+# development
+$ npm run dev
+
+# watch mode
+$ npm run dev:watch
+
+# debug mode
+$ npm run dev:debug
+
+# production mode
+$ npm run start
+```
+
+## Test
+
+```bash
+# unit tests
+$ npm run test
+
+# unit tests debug mode
+$ npm run test:debug
+
+# test coverage
+$ npm run test:cov
+```
+
+## Development
+
+### Linter
+Run `npm run lint` to make sure the code base follows configured linter rules.