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src/core/ext/README.md

 Optional plugins for gRPC Core: Modules in this directory extend gRPC Core in
-useful ways.
+useful ways. All optional code belongs here.
 
 NOTE: The movement of code between lib and ext is an ongoing effort, so this
 directory currently contains too much of the core library.

src/core/ext/resolver/README.md

 # Resolver
 
 Implementations of various name resolution schemes.
+See the [naming spec](/doc/naming.md).

src/core/lib/README.md

 Required elements of gRPC Core: Each module in this directory is required to
-build gRPC.
+build gRPC. If it's possible to envisage a configuration where code is not
+required, then that code belongs in ext/ instead.
 
 NOTE: The movement of code between lib and ext is an ongoing effort, so this
 directory currently contains too much of the core library.