Add docs about github tokens for cherrypicking

Recently, I performed a cherry pick for the first time. When I ran ./hack/cherry_pick_pull.sh
after setting up all the env variables, I was asked for my github password. When I gave my actual
github password it threw a 404 not found which confused me. I then came across this
mislav/hub#2655 (comment) . I created a github token
post which I was able to cherry pick successfully.

The hub project team is looking to fix it but until then I feel that we should document it.

Do let me know if I did something wrong here in the cherry pick process.

contributors/devel/sig-release/cherry-picks.md

@@ -27,6 +27,7 @@ branches.
 - Have `hub` installed, which is most easily installed via
   `go get github.com/github/hub` assuming you have a standard golang
   development environment.
+- A github token which has permissions to create a PR in an upstream branch.
 
 ## What Kind of PRs are Good for Cherry Picks
 
@@ -89,6 +90,9 @@ patch release branches.
     release you want to cherry pick to. Cherry picks should be applied to all
     [active](https://github.com/kubernetes/sig-release/blob/master/releases/patch-releases.md#detailed-release-history-for-active-branches)
     release branches where the fix is applicable.
+
+  - When hub intimates you for your github password, provide the created github token rather than your actual github password. 
+
 
 - Your cherry pick PR will immediately get the
   `do-not-merge/cherry-pick-not-approved` label.