docs(l1): add quick guide on how to sync with holesky (#2485)

**Motivation**
Add instructions on how to set up ethrex along with a consensus node and
start syncing with holesky or other testnets
<!-- Why does this pull request exist? What are its goals? -->

**Description**

<!-- A clear and concise general description of the changes this PR
introduces -->

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

Closes #issue_number

---------

Co-authored-by: Javier Rodríguez Chatruc <[email protected]>

Author: fmoletta

README.md

@@ -248,11 +248,11 @@ P2P options:
 
       --syncmode <SYNC_MODE>
           Can be either "full" or "snap" with "full" as default value.
-
+          
           [default: full]
 
       --p2p.enabled
-
+          
 
       --p2p.addr <ADDRESS>
           [default: 0.0.0.0]
@@ -262,12 +262,12 @@ P2P options:
 
       --discovery.addr <ADDRESS>
           UDP address for P2P discovery.
-
+          
           [default: 0.0.0.0]
 
       --discovery.port <PORT>
           UDP port for P2P discovery.
-
+          
           [default: 30303]
 
 RPC options:
@@ -285,21 +285,72 @@ RPC options:
 
       --authrpc.addr <ADDRESS>
           Listening address for the authenticated rpc server.
-
+          
           [default: localhost]
 
       --authrpc.port <PORT>
           Listening port for the authenticated rpc server.
-
+          
           [default: 8551]
 
       --authrpc.jwtsecret <JWTSECRET_PATH>
           Receives the jwt secret used for authenticated rpc requests.
-
+          
           [default: jwt.hex]
 ```
 <!-- END_CLI_HELP -->
 
+### Syncing with Holesky
+
+#### Step 1: Set up a jwt secret for both clients
+
+As an example, we put the secret in a `secrets` directory in the home folder.
+
+```bash
+mkdir -p ~/secrets
+openssl rand -hex 32 | tr -d "\n" | tee ~/secrets/jwt.hex
+```
+
+We will pass this new file’s path as an argument for both clients.
+
+#### Step 2: Launch Ethrex
+
+Pass holesky as a network and the jwt secret we set in the previous step.
+This will launch the node in full sync mode, in order to test out snap sync you can add the flag `--syncmode snap`.
+
+```bash
+cargo run --release --bin ethrex -- --http.addr 0.0.0.0 --network holesky --authrpc.jwtsecret ~/secrets/jwt.hex
+```
+
+#### Step 3: Set up a Consensus Node
+
+For this quick tutorial we will be using lighthouse, but you can learn how to install and run any consensus node by reading their documentation.
+
+You can choose your preferred installation method from [lighthouse's installation guide](https://lighthouse-book.sigmaprime.io/installation.html) and then run the following command to launch the node and sync it from a public endpoint
+
+```bash
+lighthouse bn --network holesky --execution-endpoint http://localhost:8551 --execution-jwt ~/secrets/jwt.hex --http --checkpoint-sync-url https://checkpoint-sync.holesky.ethpandaops.io
+```
+
+When using lighthouse directly from its repository, replace `lighthouse bn` with `cargo run --bin lighthouse -- bn`
+
+Aside from holesky, these steps can also be used to connect to other supported networks by replacing the `--network` argument by another supported network and looking up a checkpoint sync endpoint for that network [here](https://eth-clients.github.io/checkpoint-sync-endpoints/)
+
+If you have a running execution node that you want to connect to your ethrex node you can do so by passing its enode as a bootnode using the `--bootnodes` flag
+
+Once the node is up and running you will be able to see logs indicating the start of each sync cycle along with from which block hash to which block hash we are syncing. You will also get regular logs with the completion rate and estimated finish time for state sync and state rebuild processes during snap sync. This will look something like this:
+
+```bash
+INFO ethrex_p2p::sync: Syncing from current head 0xb5f7…bde4 to sync_head 0xce96…fa5e
+INFO ethrex_p2p::sync::state_sync: Downloading state trie, completion rate: 68%, estimated time to finish: 1h20m14s
+INFO ethrex_p2p::sync::trie_rebuild: State Trie Rebuild Progress: 68%, estimated time to finish: 1h5m45s
+```
+
+If you want to restart the sync from the very start you can do so by wiping the database using the following command:
+```bash
+cargo run --bin ethrex -- removedb
+```
+
 # ethrex L2
 
 In this mode, the ethrex code is repurposed to run a rollup that settles on Ethereum as the L1.