Merge "extensions/README.md: add sdk ext v3" into tm-dev am: b63ec41f59 am: 230cab1d4e

Original change: https://googleplex-android-review.googlesource.com/c/platform/prebuilts/sdk/+/18031647

Change-Id: I29759b42e94f33387ab8cd34b72ec6f000b95178
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>