supersize: Update README.md with technical details.

tools/binary_size/libsupersize/testdata/Diff_NullDiff.golden

Index: tools/binary_size/libsupersize/testdata/Diff_NullDiff.golden
diff --git a/tools/binary_size/libsupersize/testdata/Diff_NullDiff.golden b/tools/binary_size/libsupersize/testdata/Diff_NullDiff.golden
index 5e48003fcc041476f3e5e3998e29e345f2bceba6..8c04fa7e45c170af5bdf462408771871d64650ee 100644
--- a/tools/binary_size/libsupersize/testdata/Diff_NullDiff.golden
+++ b/tools/binary_size/libsupersize/testdata/Diff_NullDiff.golden
@@ -9,19 +9,19 @@ Common Metadata:
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