README: Add links to PDKs

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>
Signed-off-by: Johan 'proppy' Euphrosine <proppy@google.com>

README.rst

diff --git a/README.rst b/README.rst
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+GlobalFoundries Open Source PDKs
+================================
+
+This repository contains pointers to GlobalFoundries’ open source PDKs
+maintained by Google.
+
+GF180MCU
+--------
+
+https://github.com/google/gf180mcu-pdk