Add receive-string documentation page

docs/reference/radio/receive-string.md

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+# Receive String
+
+Reads the next radio packet if any and returns the first string.
+
+## Important Security Consideration
+
+The functions in the ``radio`` namespace allow the BBC micro:bit to communicate with other micro:bits.
+
+This API does not contain any form of encryption, authentication or authorization. It's purpose is solely for use as a teaching aid to demonstrate how simple communications operates, and to provide a sandpit through which learning can take place.
+
+For serious applications, BLE should be considered a substantially more secure alternative.
+
+```sig
+radio.receiveString()
+```
+
+### Return value
+
+* the first [string](/reference/types/string) of the packet if any. ```""``` otherwise.
+
+### Examples
+
+Read the string broadcasted by other micro:bits and display it.
+
+```blocks
+radio.onDataReceived(() => {
+    basic.showString(radio.receiveString());
+});
+```
+
+A simple program to send whether you are happy, or sad over ```radio```, using the A or B button to select an emotion.
+
+```blocks
+let data: string = "";
+input.onButtonPressed(Button.A, () => {
+    radio.sendString("H");
+});
+input.onButtonPressed(Button.B, () => {
+    radio.sendString("S");
+});
+radio.onDataReceived(() => {
+    data = radio.receiveString();
+    if ("H" == data) {
+        basic.showLeds(`
+            . . . . .
+            . # . # .
+            . . . . .
+            # . . . #
+            . # # # .
+            `);
+    } else if ("S" == data) {
+        basic.showLeds(`
+            . . . . .
+            . # . # .
+            . . . . .
+            . # # # .
+            # . . . #
+            `);
+    } else {
+        basic.showString("?");
+    }
+});
+```
+
+### See also
+
+[send string](/reference/input/send-string), [on data received](/reference/radio/on-data-received)