docs(README): Add usage_example in README

Signed-off-by: Bobo Bäck Engström <bobo@id8-engineering.io>

Author: Dexter9532

README.md

@@ -37,3 +37,31 @@ Build package:
 ```sh
 uv build
 ```
+
+### Usage
+
+Below is minimal example of how to use the Em511 driver
+
+```py
+from em511 import Em511
+from pymodbus.client import ModbusSerialClient
+
+client = ModbusSerialClient(
+    port="COM3",
+    baudrate=9600,
+    parity="E",
+    stopbits=1,
+    bytesize=8,
+    timeout=1,
+)
+
+device_address = 1
+
+meter = Em511(device_address=device_address, client=client)
+
+Voltage = meter.V
+meter.password = 1234
+```
+```
+
+