README: Add links to PDKs

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>
Signed-off-by: Johan 'proppy' Euphrosine <proppy@google.com>
diff --git a/README.rst b/README.rst
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+Open Source PDKs
+================
+
+This repository contains pointers to open source PDKs maintained by
+Google.
+
+Foundries
+---------
+
+SkyWater Technologies
+~~~~~~~~~~~~~~~~~~~~~
+
+SKY130
+^^^^^^
+
+https://github.com/google/skywater-pdk
+
+GlobalFoundries
+~~~~~~~~~~~~~~~
+
+https://github.com/google/globalfoundries-pdks
+
+GF180MCU
+^^^^^^^^
+
+https://github.com/google/gf180mcu-pdk