Extend all to more resources

Added more things from the list here:
https://github.com/kubernetes/kubernetes/blob/master/pkg/kubectl/cmd/cmd.go#L159

Update the devel/kubectl-conventions.md with the rules mentioned by
a few folks on which resources could be added to the special 'all' alias

kubectl-conventions.md

@@ -43,6 +43,7 @@ Updated: 8/27/2015
   - [Principles](#principles)
   - [Command conventions](#command-conventions)
     - [Create commands](#create-commands)
+    - [Rules for extending special resource alias - "all"](#rules-for-extending-special-resource-alias---all)
   - [Flag conventions](#flag-conventions)
   - [Output conventions](#output-conventions)
   - [Documentation conventions](#documentation-conventions)
@@ -118,6 +119,21 @@ creating tls secrets. You create these as separate commands to get distinct
 flags and separate help that is tailored for the particular usage.
 
 
+### Rules for extending special resource alias - "all"
+
+Here are the rules to add a new resource to the `kubectl get all` output.
+
+* No cluster scoped resources
+
+* No namespace admin level resources (limits, quota, policy, authorization
+rules)
+
+* No resources that are potentially unrecoverable (secrets and pvc)
+
+* Resources that are considered "similar" to #3 should be grouped
+the same (configmaps)
+
+
 ## Flag conventions
 
 * Flags are all lowercase, with words separated by hyphens