Add start of Unicode design doc

Author: hoelzro

dev/UNICODE.md

@@ -0,0 +1,10 @@
+# ack 2 Unicode support
+
+  - File encoding (what do we do now? what if someone tries an ack UCS-2-file?)
+  - ARGV encoding
+  - Document new regex-fu that came with 5.14?
+  - Filename/pattern normalization
+  - Output stream encoding
+  - Collation level is something I mentioned
+  - I18N::Langinfo
+  - Coloration + unicode chars