Add Solax X1 modbus example (Closes: #31)

[syssi]

• [x] Add instructions how to enable the modbus interface

-> Settings
-> Pin: 6868
-> Device management
-> Remote setting
-> Enable modbus
-> Modbuss address `1`

See https://github.com/syssi/esphome-modbus-solax-x1/issues/31

[syssi]

I'm unsure about the click path at the on-screen menu. I added a generic request to enable modbus at the YAML.