Added notes on how to fix pulling errors

Getting this error when running `helm init` locally:

    Failed to pull image "gcr.io/kubernetes-helm/tiller:v2.7": rpc error:
    code = Unknown desc = Error response from daemon: manifest for
    gcr.io/kubernetes-helm/tiller:v2.7 not found

docs/developers.md

@@ -87,7 +87,19 @@ GCR registry.
 For development, we highly recommend using the
 [Kubernetes Minikube](https://github.com/kubernetes/minikube)
 developer-oriented distribution. Once this is installed, you can use
-`helm init` to install into the cluster.
+`helm init` to install into the cluster. Note that version of tiller you're using for
+development may not be available in Google Cloud Container Registry. If you're getting
+image pull errors, you can override the version of Tiller. Example:
+
+```console
+helm init --tiller-image=gcr.io/kubernetes-helm/tiller:2.7.2
+```
+
+Or use the latest version:
+
+```console
+helm init --canary-image
+```
 
 For developing on Tiller, it is sometimes more expedient to run Tiller locally
 instead of packaging it into an image and running it in-cluster. You can do