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

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

Change-Id: Ibb42d9ea55d5cde9c03c0f04c2c377268c672a45
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>