Added bitcoin explorer

bitcoin-explorer/README.md

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+# Bitcoin Explorer
+
+The Bitcoin Explorer consists of several Substreams modules showcasing the most basic operations that you can perform with Substreams on the Bitcoin 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 an [authentication token](https://substreams.streamingfast.io/reference-and-specs/authentication).
+
+## Modules
+
+The modules in this repository answer some interesting questions when developing a blockchain application:
+
+### How Can You Get the Basic Information of a Block?
+
+For every block, the `map_block_meta` module retrieves the most relevant information of the block (number, hash, and parent hash).
+
+### How Can You Retrieve Transactions
+
+For every block, the `map_transactions` modules summaries transaction number of vin and vout, the total valut of btc sent 
+
+
+## Running the Substreams
+
+First, generate the Protobuf code, which are the outputs of the Substreams:
+
+```
+> make protogen
+```
+
+Then, build the Rust code using the `cargo` command-line tool:
+
+```
+> make build
+``` 
+
+Now, you are ready to run the Substreams. The Substreams contained in this project are independent of each other, so you must specify which Substreams module you want to run.
+
+### Running the "map_block_meta" Module
+
+In the following command, you retrieve the metadata of the block with number `819113`. You specify the starting block by using the `--start-block` parameter.
+
+```bash
+> substreams run -e mainnet.btc.streamingfast.io:443 substreams.yaml map_block_meta --start-block 819113 --stop-block +1
+Connected (trace ID dabf2d0d5f8074a36da855579279e23c)
+Progress messages received: 0 (0/sec)
+Backprocessing history up to requested target block 819113:
+(hit 'm' to switch mode)
+
+
+
+
+
+----------- BLOCK #819,113 (00000000000000000003aefe02ab07a4a7c5a27bb674b062732286a71ab1ab51) ---------------
+{
+  "@module": "map_block_meta",
+  "@block": 819113,
+  "@type": "btc.block_meta.v1.BlockMeta",
+  "@data": {
+    "number": "819113",
+    "hash": "00000000000000000003aefe02ab07a4a7c5a27bb674b062732286a71ab1ab51",
+    "parentHash": "000000000000000000001be3e5929be506d4e2c8cd1b7ba4fa9422032434368f"
+  }
+}
+
+all done
+```