doc/go1.5: mention new SWIG requirement: must be 3.0.6 or later.

Change-Id: I18b78677b2ac9a82560d5d787225d4fc46d451d3
Reviewed-on: https://go-review.googlesource.com/7050Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.5.txt

@@ -26,6 +26,7 @@ testing/quick: support generation of arrays (https://golang.org/cl/3865)
 Tools:
 
 cmd/go: std wildcard now excludes commands in main repo (https://golang.org/cl/5550)
+cmd/go: .swig/.swigcxx files now require SWIG 3.0.6 or later
 cmd/vet: better validation of struct tags (https://golang.org/cl/2685)
 cmd/ld: no longer record build timestamp in Windows PE file header (https://golang.org/cl/3740)