Clarify caching behavior and dependency vendoring in readme.
Clarify caching behavior and dependency vendoring in readme.

PiperOrigin-RevId: 307847171
Change-Id: I703c21ba83e6fd7ca0efd80d54e68d5bae131b34

GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks