docs: add readme for writing operations

[zhongys-c8y]

Added a section and two screenshots for writing operations to explain how to trigger operations on Cumulocity correctly

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
22	0	0	22	100	39.308590999s

Passed Tests

Name	⏱️ Duration	Suite
Device should support the operation c8y_ModbusConfiguration	0.104 s	Device
Set values via c8y_ModbusConfiguration Operation	2.339 s	Operation
Poll rate and transmit rate should be updated for the Device	0.217 s	Operation
Poll rate and transmit rate should be updated on the Device	2.330 s	Operation
Device should support the operation c8y_ModbusDevice	0.108 s	Device
Device should support the operation c8y_SerialConfiguration	0.106 s	Device
Set values via c8y_SerialConfiguration Operation	2.327 s	Operation
Serial configuration should be updated for the Device	0.197 s	Operation
Serial configuration should be updated on the Device	2.329 s	Operation
Device should have installed software tedge-modbus-plugin	0.130 s	Debian
Service should be active	2.324 s	Debian
ReInstall Modbus Plugin	20.588 s	Debian
Device should have the fragment c8y_ModbusConfiguration	0.211 s	Device
ChildDevice TestCase1 should be created	0.203 s	Device
ChildDevice TestCase1 should have the fragment c8y_ModbusDevice	0.503 s	Device
Service tedge-modbus-plugin should be enabled	0.105 s	Device
Device should set supported operations for writing to registers and coils	0.100 s	Operations
Device should write to a registers	2.343 s	Operations
Device should write to a coil	2.336 s	Operations
ChildDevice TestCase1 should have a Test.Int16 Measurement	0.204 s	Telemetry
ChildDevice TestCase1 should have a Test.Float32 Measurement	0.204 s	Telemetry
ChildDevice TestCase1 should have Alarms of type TestAlarm on Coil Value 1	0.001 s	Telemetry