doc: document go get -f flag in 1.4 release notes

LGTM=r, rsc
R=r, rsc, adg
CC=golang-codereviews
https://golang.org/cl/168890043

doc/go1.4.html

@@ -371,6 +371,15 @@ fails because of this check, the mis-imported package has been copied to the loc
 and should be removed manually.
 </p>
 
+<p>
+To complement this new feature, a check has been added at update time to verify
+that the local package's remote repository matches that of its custom import.
+The <code>go</code> <code>get</code> <code>-u</code> command will fail to
+update a package if its remote repository has changed since it was first
+downloaded.
+The new <code>-f</code> flag overrides this check.
+</p>
+
 <p>
 Further information is in
 <a href="http://golang.org/s/go14customimport">the design document</a>.