Add a README to frameworks/base/config.

Bug: 174746397
Test: N/A
Change-Id: Ie1131c9ff818d1a5308774eb51337f8200a85d79

config/README.md

+# Configuration files for ART compiling the framework
+
+* boot-image-profile.txt: A list of methods from the boot classpath to be compiled by dex2oat.
+  The order in the file is not relelvant.
+* boot-profile.txt: An ordered list of methods from the boot classpath to be compiled by
+  the JIT in the order provided in the file. Used by JIT zygote, when on-device
+  signing failed.
+* dirty-image-objects: List of objects in the boot image which are known to
+  become dirty. This helps binning objects in the image file.
+* preloaded-classes: classes that will be allocated in the boot image, and
+  initialized by the zygote.
+* preloaded-classes-denylist: Classes that should not be initialized in the
+  zygote, as they have app-specific behavior.