Add long click instructions to readme am: 32063241b3

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/Car/RotaryController/+/22777076

Change-Id: If0f5715d3478220aa9b39db1123194f3957d3d96
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>
diff --git a/readme.md b/readme.md
index ad5745d..30902d7 100644
--- a/readme.md
+++ b/readme.md
@@ -43,3 +43,9 @@
 ```
 adb shell cmd car_service inject-key 23
 ```
+
+To long click the controller center button, send down and up action seperately. For example:
+```
+adb shell cmd car_service inject-key 23 -a down && sleep 2 && adb shell cmd car_service inject-key 23 -a up
+```
+