add beacon chain explorer

beacon-explorer/README.md

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+# Beacon Explorer
+
+The Ethereum Beacon Chain Explorer consists of several Substreams modules showcasing the most basic operations that you can perform with Substreams on the Ethereum Beacon blockchain.
+
+## Before You Begin
+
+Make sure you have the [Substreams CLI installed](https://substreams.streamingfast.io/getting-started/installing-the-cli), and you know the [basic structure of a Substreams module](https://substreams.streamingfast.io/getting-started/quickstart). You must also provide a Pinax [API key](https://app.pinax.network) as `$SUBSTREAMS_API_KEY` environment variable
+
+## Modules
+
+The substreams package in this repository provides the following substreams modules:
+- `map_block_meta` - streams some block metadata, such as slot number, root hash, spec version, timestamp, etc
+- `map_block_full` - streams the entire block content to help you explore available fields
+- `map_blobs` - streams blobs embedded into the beacon chain block
+
+
+## Building the Substreams
+
+First, generate the Protobuf code, which is the output of the Substreams:
+
+```
+> make protogen
+```
+
+Then, build the Rust code using the `cargo` command-line tool:
+
+```
+> make build
+```
+
+## Running Substreams modules
+
+And now you can stream the corresponding substreams modules:
+
+```
+> make run map_block_meta
+> make run map_block_full
+> make run map_blobs
+```
+
+Alternatively, you can take advantage of `substreams gui` command for a nicer user interface:
+
+```
+> make gui map_block_meta
+> make gui map_block_full
+> make gui map_blobs
+```