doc: add mime/quotedprintable to go1.5.txt

Change-Id: Iace8941c947253b1141f4194c5d2010c420ec220
Reviewed-on: https://go-review.googlesource.com/8540Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.5.txt

@@ -32,6 +32,7 @@ go/ast: add Implicit field to ast.EmptyStmt; changed meaning of ast.EmptyStmt.Se
 log: add SetOutput functions (https://golang.org/cl/2686, https://golang.org/cl/3023)
 math/big: add arbitrary precision Floats (many cl's)
 mime: add ExtensionByType (https://golang.org/cl/7444)
+mime/quotedprintable: new package (https://golang.org/cl/5940 + others)
 net/http: support for setting trailers from a server Handler (https://golang.org/cl/2157)
 net/http: ignore the Unix epoch time in ServeContent (https://golang.org/cl/7915)
 net/http/cgi: fix REMOTE_ADDR, REMOTE_HOST, add REMOTE_PORT (https://golang.org/cl/4933)