QSPI: add contributing guide

Author: 0xc0170

docs/reference/contributing/target/QuadSPI.md

@@ -1,25 +1,28 @@
 <h3 id="quadspi-port">QuadSPI</h3>
 
-[Include a brief description here.]
+QSPI HAL defines API for targets that contain QSPI capable peripheral. The QSPI interface is often used for data storage.
 
 #### Assumptions
 
 ##### Defined behavior
 
-[Include any defined behavior in bullet format here.]
+- a target implementaion should cover the most of QSPI frame format (some targets might not provide the flexibility for setting all frame parameters)
+- command transfer - a target might provide additional API for sending device specific commands. In case it does not, it can be implemented via read/write functions
 
 ##### Undefined behavior
 
-[Include any undefined behavior in bullet format here.]
+- Calling any function other than `qspi_init` before the initialization of the QSPI
 
-##### Potential bugs
+#### Dependency
 
-[Include any potential bugs in bullet format here.]
+QSPI peripheral
 
 #### Implementing QuadSPI
 
-[Include implementation information here.]
+The target needs to define `qspi_s` structure - target specific QSPI object, enable QSPI in targets.json file `device_has` and implement QSPI HAL functions defined in `hal/qspi_api.h` header file.
+
+`qspi_write` and `qspi_read` are used for data transfers. For communicating with device, `qspi_command_transfer` should be used.
 
 #### Testing
 
-[Include testing information here.]
+To be implemented