Update SMP branch readme for port migration (#999)

Author: chinglee-iot

README.md

@@ -9,6 +9,44 @@ The contents of this branch will remain available for certain period but we will
 Have more questions? Post them in the [FreeRTOS forum](https://forums.freertos.org/).
 
 
+## Migrate port from SMP branch to FreeRTOS v11
+
+The following changes should be applied to migrate a port from this branch to FreeRTOS v11:
+
+* Call `xTaskIncrementTick` in critical section in port
+
+RP2040 example:
+```c
+void xPortSysTickHandler( void )
+{
+    portBASE_TYPE xPreviousMask;
+
+    /* xTaskIncrementTick now must be called in critical section. */
+    xPreviousMask = taskENTER_CRITICAL_FROM_ISR();
+    {
+        /* Increment the RTOS tick. */
+        if( xTaskIncrementTick() != pdFALSE )
+        {
+            /* Pend a context switch. */
+            portNVIC_INT_CTRL_REG = portNVIC_PENDSVSET_BIT;
+        }
+    }
+    taskEXIT_CRITICAL_FROM_ISR( xPreviousMask );
+}
+```
+
+* Rename `configNUM_CORES` to `configNUMBER_OF_CORES`
+
+* Define `portSET/CLEAR_INTERRUPT_MASK` in port
+
+* Define `portENTER/EXIT_CRITICAL_FROM_ISR` for SMP in port
+    * These macros should be implemented with `vTaskEnterCriticalFromISR`/`xTaskExitCriticalFromISR`
+ 
+* Update `portSET/CLEAR_INTERRUPT_MASK_FROM_ISR` implementation in port
+    * SMP-dev doesn’t use these macros to enter/exit critical section from ISR. Instead,
+ `portENTER/EXIT_CRITICAL_FROM_ISR` are used. These functions should be implemented as
+ the macro name suggested, set or clear interrupt mask from ISR if nested interrupt are supported.
+
 
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