Document OTG mode in README

PR #2974 <https://github.com/Genymobile/scrcpy/pull/2974>

README.md

@@ -35,6 +35,7 @@ Its features include:
    (Linux-only)
  - [physical mouse simulation (HID)](#physical-mouse-simulation-hid)
    (Linux-only)
+ - [OTG mode](#otg) (Linux-only)
  - and more…
 
 ## Requirements
@@ -849,6 +850,26 @@ Special capture keys, either <kbd>Alt</kbd> or <kbd>Super</kbd>, toggle
 the mouse back to the computer.
 
 
+#### OTG
+
+It is possible to run _scrcpy_ with only physical keyboard and mouse simulation
+(HID), as if the computer keyboard and mouse were plugged directly to the device
+via an OTG cable.
+
+In this mode, _adb_ (USB debugging) is not necessary, and mirroring is disabled.
+
+To enable OTG mode:
+
+```bash
+scrcpy --otg
+# Pass the serial if several USB devices are available
+scrcpy --otg -s 0123456789abcdef
+```
+
+Like `--hid-keyboard` and `--hid-mouse`, it only works if the device is
+connected by USB, and is currently only supported on Linux.
+
+
 #### Text injection preference
 
 There are two kinds of [events][textevents] generated when typing text: