Update the existing salt.md, add the start of a Salt README tree

Starts to fix #6070

cluster/saltbase/README.md

@@ -0,0 +1,17 @@
+# SaltStack configuration
+
+This is the root of the SaltStack configuration for Kubernetes. A high
+level overview for the Kubernetes SaltStack configuration can be found [in the docs tree.](../../docs/salt.md)
+
+This SaltStack configuration currently applies to default
+configurations for Debian-on-GCE, Fedora-on-Vagrant, Ubuntu-on-AWS and
+Ubuntu-on-Azure. (That doesn't mean it can't be made to apply to an
+arbitrary configuration, but those are only the in-tree OS/IaaS
+combinations supported today.) As you peruse the configuration, these
+are shorthanded as `gce`, `vagrant`, `aws`, `azure` in `grains.cloud`;
+the documentation in this tree uses this same shorthand for convenience.
+
+See more:
+* [pillar](pillar/)
+* [reactor](reactor/)
+* [salt](salt/)