added readme

usermods/pwm_outputs/readme.md

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+# PWM outputs
+
+v2 Usermod to add generic PWM outputs to WLED. Usermode could be used to control servo motors, LED brightness or any other device controlled by PWM signal.
+
+## Installation
+
+Add the compile-time option `-D USERMOD_PWM_OUTPUTS` to your `platformio.ini` (or `platformio_override.ini`). By default upt to 3 PWM outputs could be configured, to increase that limit add build argument `-D USERMOD_PWM_OUTPUT_PINS=10` (replace 10 by desired amount).
+
+Currently only ESP32 is supported.
+
+## Configuration
+
+By default PWM outputs are disabled, navigate to Usermods settings and configure desired PWM pins and frequencies.
+
+## Usage
+
+If PWM output is configured, it starts to publish its duty cycle value (0-1) both to state JSON and to info JSON (visible in UI info panel). To set PWM duty cycle, use JSON api (over HTTP or over Serial)
+
+```json
+{"pwm_0": 0.0, "pwm_1": 0.1, ...}
+```