add readme for reference dir

diff --git a/src/nmigen_gf/reference/README.mdwn b/src/nmigen_gf/reference/README.mdwn
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+++ b/src/nmigen_gf/reference/README.mdwn
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+# Python Reference Code for Carry-Less and Galois Field Operations
+
+Stored here: <https://git.libre-soc.org/?p=nmigen-gf.git;a=tree;f=src/nmigen_gf/reference;hb=HEAD>