Document APK drag & drop

Add the drag & drop shortcut for installing an APK file to the README
and the help.

README.md

@@ -300,6 +300,7 @@ To run without installing:
  | turn screen on                         | _Right-click²_                |
  | paste computer clipboard to device     | `Ctrl`+`v`                    |
  | enable/disable FPS counter (on stdout) | `Ctrl`+`i`                    |
+ | install APK from computer              | drag & drop APK file          |
 
 _¹Double-click on black borders to remove them._  
 _²Right-click turns the screen on if it was off, presses BACK otherwise._

app/src/main.c

@@ -98,6 +98,9 @@ static void usage(const char *arg0) {
         "\n"
         "    Ctrl+i\n"
         "        enable/disable FPS counter (print frames/second in logs)\n"
+        "\n"
+        "    Drag & drop APK file\n"
+        "        install APK from computer\n"
         "\n",
         arg0,
         DEFAULT_BIT_RATE,