docs: add beacon-API to README (#958)

Makefile

@@ -113,7 +113,7 @@ grafana-clean:
 
 #▶️ start: @ Start application with Beacon API.
 start: compile-all
-	iex -S mix phx.server
+	iex -S mix run -- --beacon-api
 
 #▶️ iex: @ Runs an interactive terminal with the main supervisor setup.
 iex: compile-all

README.md

@@ -90,6 +90,22 @@ Some public endpoints can be found in [eth-clients.github.io/checkpoint-sync-end
 > The data retrieved from the URL is stored in the DB once the node is initiated (i.e. the iex prompt shows).
 > Once this happens, following runs of `make iex` will start the node using that data.
 
+### Beacon API
+
+You can start the application with the Beacon API on the default port `4000` running:
+```shell
+make start
+```
+
+You can also specify a port with the "--beacon-api-port" flag:
+
+```shell
+iex -S mix run -- --beacon-api --beacon-api-port <your_port_here>
+```
+> [!WARNING]
+> In case checkpoint-sync is needed, following the instructions above will end immediately with an error (see [Checkpoint Sync](#checkpoint-sync)).
+>
+
 ### Tests, linting and formatting
 
 Our CI runs tests, linters, and also checks formatting and typing.