add a disclaimer

SVN=126379

doc/go_lang.txt

@@ -4,13 +4,15 @@ The Go Programming Language (DRAFT)
 Robert Griesemer, Rob Pike, Ken Thompson
 
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-(July 7, 2008)
+(July 8, 2008)
 
 This document is a semi-formal specification/proposal for a new
 systems programming language.  The document is under active
 development; any piece may change substantially as design progresses;
 also there remain a number of unresolved issues.
 
+This draft document is unpublished and under active development.
+It is not ready for external review.
 
 Guiding principles
 ----