add rtos Mail example

APIs_RTOS/Mail/README.md

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+# Mail example
+
+This example shows how to use Mail for threads communication.
+

APIs_RTOS/Mail/main.cpp

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+/*
+ * Copyright (c) 2020 Arm Limited and affiliates.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#include "mbed.h"
+
+/* Mail */
+typedef struct {
+  float    voltage; /* AD result of measured voltage */
+  float    current; /* AD result of measured current */
+  uint32_t counter; /* A counter value               */
+} mail_t;
+
+Mail<mail_t, 16> mail_box;
+Thread thread;
+
+void send_thread (void) {
+    uint32_t i = 0;
+    while (true) {
+        i++; // fake data update
+        mail_t *mail = mail_box.alloc();
+        mail->voltage = (i * 0.1) * 33; 
+        mail->current = (i * 0.1) * 11;
+        mail->counter = i;
+        mail_box.put(mail);
+        ThisThread::sleep_for(1000);
+    }
+}
+
+int main (void) {
+    thread.start(callback(send_thread));
+
+    while (true) {
+        osEvent evt = mail_box.get();
+        if (evt.status == osEventMail) {
+            mail_t *mail = (mail_t*)evt.value.p;
+            printf("\nVoltage: %.2f V\n\r"   , mail->voltage);
+            printf("Current: %.2f A\n\r"     , mail->current);
+            printf("Number of cycles: %lu\n\r", mail->counter);
+
+            mail_box.free(mail);
+        }
+    }
+}