database/sql: link to wiki in package docs

Update #5886

R=golang-dev, kamil.kisiel, adg, r, rsc, dave, arnehormann, bradfitz
CC=golang-dev
https://golang.org/cl/14087043

src/pkg/database/sql/sql.go

@@ -7,6 +7,9 @@
 //
 // The sql package must be used in conjunction with a database driver.
 // See http://golang.org/s/sqldrivers for a list of drivers.
+//
+// For more usage examples, see the wiki page at
+// http://golang.org/s/sqlwiki.
 package sql
 
 import (