doc: move go1.5.txt to go1.5.html

Just a copy, other than adding a header, preparatory to preparing the release notes.

Change-Id: Ia4dc27777b96b1e898d9873e7d38e6e795057698
Reviewed-on: https://go-review.googlesource.com/11474Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.5.txt → doc/go1.5.html

+<!--{
+	"Title": "Go 1.5 Release Notes",
+	"Path":  "/doc/go1.5",
+	"Template": true
+}-->
+
+<pre>
 Overall:
 - toolchain in Go
 - new GC
@@ -159,3 +166,4 @@ Standard library hardening
 were fixed in fmt, archive/zip, archive/tar, encoding/gob, image/jpeg, image/png,
 image/gif, compress/flate, text/template, html/template. The fixes harden implementation
 against incorrect and malicious inputs.
+</pre>