Add README.md with source check out instructions

Change-Id: I6c84ae5585eaa562a43827a162ab11adee83bd9d
diff --git a/README.md b/README.md
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+# Hafnium
+
+``` shell
+mkdir hafnium
+cd hafnium
+repo init -u sso://hafnium/manifest
+repo sync -j4
+```