Move NIE spec to a new doc/spec directory
diff --git a/doc/spec/README.md b/doc/spec/README.md
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+This directory contains various specifications, whether intrinsically related to
+Wuffs or stand-alone formats that were invented alongside Wuffs' development.
+
+ - [Naïve Image Formats: NIE, NII, NIA](./nie-spec.md)
diff --git a/lib/nie/README.md b/doc/spec/nie-spec.md
similarity index 100%
rename from lib/nie/README.md
rename to doc/spec/nie-spec.md
