docs: add metadata readme

packages/metadata/README.md

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+# ZKchainHub - Metadata Package
+
+## Overview
+
+This package contains providers for fetching chains and tokens metadata.
+
+### chains.json
+
+This file contains an array of Chain metadata. To add a new one, follow the following interface:
+
+```json
+{
+    "chainId": 324, //mandatory
+    "name": "ZKsyncERA", //mandatory
+    "iconUrl": "https://s2.coinmarketcap.com/static/img/coins/64x64/24091.png", //optional
+    "publicRpcs": [
+        "https://mainnet.era.zksync.io",
+        "https://zksync.drpc.org",
+        "https://zksync.meowrpc.com"
+    ], //optional,
+    "explorerUrl": "https://explorer.zksync.io/", //optional
+    "launchDate": 1679626800, //mandatory
+    "chainType": "Rollup", // "Rollup" | "Validium"
+    "baseToken": {
+        "name": "Ether", //mandatory
+        "symbol": "ETH", //mandatory
+        "type": "native", // "native" | "erc20"
+        "contractAddress": null, // null if "native", address if "erc20"
+        "imageUrl": "https://coin-images.coingecko.com/coins/images/279/large/ethereum.png?1696501628", //optional
+        "decimals": 18 //mandatory
+    }
+}
+```
+
+### tokens.json
+
+This file contains an array of Token metadata. To add a new one, follow the following interface:
+
+```json
+{
+    "name": "0x Protocol Token", //mandatory
+    "symbol": "ZRX", //mandatory
+    "contractAddress": "0xE41d2489571d322189246DaFA5ebDe1F4699F498", // null if "native", address if "erc20"
+    "imageUrl": "https://assets.coingecko.com/coins/images/863/large/0x.png?1696501996", //optional
+    "type": "erc20", // "native" | "erc20"
+    "decimals": 18 //mandatory
+}
+```
+
+Currently, there are three different providers:
+
+-   `LocalFileMetadataProvider`
+-   `GithubMetadataProvider`
+-   `StaticMetadataProvider`
+
+Inside [examples](./examples/) folder, you'll find json examples. Copy it to your local machine and edit metadata as wanted. Also, you can use it to host your own file on Github (recommended)
+
+At [ZKchainHub-metadata repository](https://github.com/defi-wonderland/ZKchainHub-metadata) you'll find the latest curated list of tokens. To use it, remember to copy file url as raw content.
+
+> https://raw.githubusercontent.com/defi-wonderland/ZKchainHub-metadata/79779a6313ab43af055f59861be012bf67bb908d/chains_mainnet.json
+
+## 📋 Prerequisites
+
+-   Ensure you have `node >= 20.0.0` and `pnpm >= 9.5.0` installed.
+
+## Installation
+
+```bash
+$ pnpm install
+```
+
+## Building
+
+To build the monorepo packages, run:
+
+```bash
+$ pnpm build
+```
+
+## Test
+
+```bash
+# unit tests
+$ pnpm run test
+
+# test coverage
+$ pnpm run test:cov
+```
+
+## Contributing
+
+To create a new provider, create it inside [`providers`](./src/providers/) folder and implement the [`IMetadataProvider`](./src/interfaces/metadata.interface.ts) interface.
+
+Then, write the configuration interface inside [`metadataConfig.interface.ts`](./src/interfaces/metadataConfig.interface.ts) and add the provider to the [`MetadataProviderFactory`](./src/factory/index.ts) class.
+
+Finally, export the provider and required files in [`external.ts`](./src/external.ts).