QSPI: add contributing guide

docs/reference/contributing/target/QuadSPI.md

@@ -1,25 +1,56 @@
 <h2 id="quadspi-port">QuadSPI</h2>
 
-[Include a brief description here.]
+Implementing QSPI enables Mbed OS to communicate with external memories much faster than via SPI. Because the communication can be up to four lines between the host and a device.
+
+The most common use case is an external memory to use as additional data storage.
+
+<span class="warnings">**Warning:** We are changing the QSPI HAL API in an upcoming release of Mbed OS. You can find details on how it may affect you in the [Implementing the QSPI API](#implementing-the-qspi-api) section.
 
 ### Assumptions
 
 #### Defined behavior
 
-[Include any defined behavior in bullet format here.]
+- A target implementaion covers most of the QSPI frame format (some targets might not provide the flexibility for setting all frame parameters).
+- Command transfer - A target might provide additional functions for sending device-specific commands. If it does not, you can implement it using read and write functions. (This is target or driver dependent.)
 
 #### 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.]
+To make sure your platform is ready for the upcoming changes, you need to implement the future API and submit it in a separate pull request. You can find the API and specification for the new QuadSPI API in the following header file:
+
+[![View code](https://www.mbed.com/embed/?type=library)](https://os-doc-builder.test.mbed.com/docs/development/feature-hal-spec-qspi-doxy/classmbed_1_1_q_s_p_i.html)
+
+The target needs to define the `qspi_s` structure - target specific QSPI object.
+
+Functions to implement:
+
+```
+qspi_status_t qspi_init(qspi_t *obj, PinName io0, PinName io1, PinName io2, PinName io3, PinName sclk, PinName ssel, uint32_t hz, uint8_t mode);
+qspi_status_t qspi_free(qspi_t *obj);
+qspi_status_t qspi_frequency(qspi_t *obj, int hz);
+qspi_status_t qspi_write(qspi_t *obj, const qspi_command_t *command, const void *data, size_t *length);
+qspi_status_t qspi_command_transfer(qspi_t *obj, const qspi_command_t *command, const void *tx_data, size_t tx_size, void *rx_data, size_t rx_size); 
+qspi_status_t qspi_read(qspi_t *obj, const qspi_command_t *command, void *data, size_t *length);
+
+```
+
+Use `qspi_write` and `qspi_read` for data transfers. For communicating with a device, use `qspi_command_transfer`.
+
+To enable the QSPI HAL, define `QSPI` in the targets.json file inside `device_has`:
+
+```
+"TARGET_NAME": {
+   "device_has": ["QSPI"]
+}
+```
 
 ### Testing
 
-[Include testing information here.]
+To be implemented