Add README for _common directory (#51335)

Signed-off-by: Edward Oakes <ed.nmi.oakes@gmail.com>

Author: edoakes

python/ray/_common/README.md

@@ -0,0 +1,7 @@
+# Common Utilities Shared Across the Libraries
+
+This directory contains logic shared across Ray Core and the native libraries.
+
+- All dependencies by the libraries on non-public APIs in the repo should live here. Libraries should _not_ depend on `ray._private`.
+- Interfaces exposed in this directory should be treated similarly to a "developer API."
+- End users and external libraries not inside the Ray repo should not depend on any code in `ray._common` (the same as `ray._private`).