RC module documentation (#2473)

docs/en/modules/rc.md

@@ -2,3 +2,45 @@
 | Since  | Origin / Contributor  | Maintainer  | Source  |
 | :----- | :-------------------- | :---------- | :------ |
 | 2015-06-12 | [Mike Wen](https://github.com/mikewen) | - | [rc.c](../../../app/modules/rc.c)|
+
+Module to generate series of impulses for remote control via 433/315Mhz radio transmitter.
+Superseded by **[rfswitch](./rfswitch.md)** module which have same functionality, and supports more transmission protocols.
+
+For more detailed description see [rfswitch module documentation](./rfswitch.md).
+
+## rc.send()
+Sends series of impulses
+
+#### Syntax
+`rc.send(pin, value, length, pulse_length, protocol_id, repeat)`
+
+which is similar to:
+`rfswitch.send(protocol_id, pulse_length, repeat, pin, value, length)`
+
+
+#### Parameters
+- `pin` 0~12, I/O index of pin, example 6 is for GPIO12, see [details](../modules/gpio/)
+- `value` positive integer value, this is the primary data which will be sent
+- `length` bit length of value, if value length is 3 bytes, then length is 24
+- `pulse_length` length of one pulse in microseconds, usually from 100 to 650
+- `protocol_id` positive integer value, from 1-3
+- `repeat` repeat value, usually from 1 to 5. This is a synchronous task. Setting the repeat count to a large value will cause problems.
+The recommended limit is about 1-4.
+
+#### Returns
+`1` always 1
+
+#### Example
+```lua
+-- Lua transmit radio code using protocol #1
+-- pulse_length 300 microseconds
+-- repeat 5 times
+-- use pin #7 (GPIO13)
+-- value to send is 560777
+-- value length is 24 bits (3 bytes)
+rc.send(7, 560777, 24, 300, 1, 5)
+```
+which is similar  to:
+```lua
+rfswitch.send(1, 300, 5, 7, 560777, 24)
+```