docs: add ERA package to readme (#8644)

**Motivation**

- The era package was not mentioned in the readme.

**Description**

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request. -->

<!-- If applicable, add screenshots to help explain your solution -->

<!-- Link to issues: Resolves #111, Resolves #222 -->


**AI Assistance Disclosure**

- [x] External Contributors: I have read the [contributor
guidelines](https://github.com/ChainSafe/lodestar/blob/unstable/CONTRIBUTING.md#ai-assistance-notice)
and disclosed my usage of AI below.

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Author: guha-rahul

README.md

@@ -51,6 +51,7 @@ yarn build
 | [`@lodestar/beacon-node`](./packages/beacon-node)           | [![npm](https://img.shields.io/npm/v/@lodestar/beacon-node)](https://www.npmjs.com/package/@lodestar/beacon-node)           | [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) | [![documentation](https://img.shields.io/badge/readme-blue)](./packages/beacon-node)      | :rotating_light: Beacon-chain client                                           |
 | [`@lodestar/config`](./packages/config)                     | [![npm](https://img.shields.io/npm/v/@lodestar/config)](https://www.npmjs.com/package/@lodestar/config)                     | [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) | [![documentation](https://img.shields.io/badge/readme-blue)](./packages/config)           | :spiral_notepad: Eth Consensus types and params bundled together               |
 | [`@lodestar/db`](./packages/db)                             | [![npm](https://img.shields.io/npm/v/@lodestar/db)](https://www.npmjs.com/package/@lodestar/db)                             | [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) | [![documentation](https://img.shields.io/badge/readme-blue)](./packages/db)               | :floppy_disk: Read/write persistent Eth Consensus data                         |
+| [`@lodestar/era`](./packages/era)                           | [![npm](https://img.shields.io/npm/v/@lodestar/era)](https://www.npmjs.com/package/@lodestar/era)                           | [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) | [![documentation](https://img.shields.io/badge/readme-blue)](./packages/era)              | :file_folder: Era file handling for historical beacon chain data               |
 | [`@lodestar/flare`](./packages/flare)                       | [![npm](https://img.shields.io/npm/v/@lodestar/flare)](https://www.npmjs.com/package/@lodestar/flare)                       | [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) | [![documentation](https://img.shields.io/badge/readme-blue)](./packages/flare)            | :boom: Command tool for triggering non-standard actions                        |
 | [`@lodestar/fork-choice`](./packages/fork-choice)           | [![npm](https://img.shields.io/npm/v/@lodestar/fork-choice)](https://www.npmjs.com/package/@lodestar/fork-choice)           | [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) | [![documentation](https://img.shields.io/badge/readme-blue)](./packages/fork-choice)      | :fork_and_knife: Beacon-chain fork choice                                      |
 | [`@lodestar/light-client`](./packages/light-client)         | [![npm](https://img.shields.io/npm/v/@lodestar/light-client)](https://www.npmjs.com/package/@lodestar/light-client)         | [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) | [![documentation](https://img.shields.io/badge/readme-blue)](./packages/light-client)     | :bird: Ethereum Light client                                                   |