Add docs for pwm led platform. (#2402)

* Add docs for pwm led platform.

* Renamed pwm platform to rpi_gpio_pwm.

source/_components/light.rpi_gpio_pwm.markdown

@@ -0,0 +1,50 @@
+---
+layout: page
+title: "PWM LED"
+description: "Instructions on how to setup pwm leds within Home Assistant."
+date: 2017-04-04 13:00
+sidebar: true
+layout: page
+comments: false
+sharing: true
+footer: true
+ha_category: Light
+ha_iot_class: "Local Push"
+ha_release: 0.43
+---
+
+The `rpi_gpio_pwm` platform allows to control multiple lights using pulse-width modulation, for example led strips. It supports one-color, RGB and RGBW leds driven by GPIOs of an Raspberry Pi or a PCA9685 controller.
+
+### {% linkable_title Configuration %}
+
+```yaml
+# Example configuration.yaml entry
+light:
+  platform: rpi_gpio_pwm
+  leds:
+    # One-color led directly connected to GPIO
+    - name: Lightstrip Cupboard
+      driver: gpio
+      pins: [17]
+      type: simple
+    # RGB led connected to PCA9685 controller
+    - name: TV Backlight
+      driver: pca9685
+      pins: [0, 1, 2] # [R, G, B]
+      type: rgb
+    # RGBW led connected to PCA9685 controller
+    - name: Lightstrip Desk
+      driver: pca9685
+      pins: [3, 4, 5, 6] # [R, G, B, W]
+      type: rgbw
+```
+
+Configuration variables:
+
+- **leds** array (*Required*):
+  - **name** (*Required*): The name of the led.
+  - **driver** (*Required*): The driver, which controls the led. Choose either `gpio` or `pca9685`.
+  - **pins** (*Required*): The pins connected to the led. The order of pins is determined by the specified type.
+  - **type** (*Required*): The type of led. Choose either `rgb`, `rgbw` or `simple`.
+  - **freq** (*Optional*): The PWM frequency. (Default: `200`)
+  - **address** (*Optional*): The address of the PCA9685 driver. (Default: `0x40`)