lib,shared: Update README's based on contents of these dirs.

Signed-off-by: Damien George <damien@micropython.org>

lib/README.md

@@ -1,2 +1,3 @@
-This directory contains standard, low-level C libraries with emphasis on
-being independent and efficient.  They can be used by any port.
+This directory contains third-party, low-level C libraries and SDKs.
+Libraries that do not target any specific platform are generally chosen
+based on them being independent and efficient.

shared/README.md

@@ -0,0 +1,3 @@
+This directory contains libraries, utilities and helper code developed
+specifically for this project.  The code is intended to be portable and
+usable by any port.