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/)

cluster/saltbase/install.sh

@@ -15,7 +15,7 @@
 # limitations under the License.
 
 # This script will set up the salt directory on the target server.  It takes one
-# argument that is a tarball with the pre-compiled kuberntes server binaries.
+# argument that is a tarball with the pre-compiled kubernetes server binaries.
 
 set -o errexit
 set -o nounset

cluster/saltbase/pillar/README.md

@@ -0,0 +1,19 @@
+The
+[SaltStack pillar](http://docs.saltstack.com/en/latest/topics/pillar/)
+data is partially statically dervied from the contents of this
+directory. The bulk of the pillars are hard to perceive from browsing
+this directory, though, because they are written into
+[cluster-params.sls](cluster-params.sls) at cluster inception.
+
+* [cluster-params.sls](cluster-params.sls) is generated entirely at cluster inception. See e.g. [configure-vm.sh](../../gce/configure-vm.sh#L226)
+* [docker-images.sls](docker-images.sls) stores the Docker tags of the current Docker-wrapped server binaries, twiddling by the Salt install script
+* [logging.sls](logging.sls) defines the cluster log level
+* [mine.sls](mine.sls): defines the variables shared across machines in the Salt
+  mine. It is starting to be largely deprecated in use, and is totally
+  unavailable on GCE, which runs standalone.
+* [privilege.sls](privilege.sls) defines whether privileged containers are allowed.
+* [top.sls](top.sls) defines which pillars are active across the cluster.
+
+## Future work
+
+Document the current pillars across providers

cluster/saltbase/reactor/README.md

@@ -0,0 +1,3 @@
+[SaltStack reactor](http://docs.saltstack.com/en/latest/topics/reactor/) files, largely defining reactions to new nodes.
+
+**Ignored for GCE, which runs standalone on each machine**

cluster/saltbase/salt/README.md

@@ -0,0 +1,34 @@
+This directory forms the base of the main SaltStack configuration. The
+place to start with any SaltStack configuration is
+[top.sls](top.sls). However, unless you are particularly keen on
+reading Jinja templates, the following tables break down what
+configurations run on what providers. (NB: The [_states](_states/)
+directory is a special directory included by Salt for `ensure` blocks,
+and is only used for the [docker](docker/) config.)
+
+Key: M = Config applies to master, n = config applies to nodes
+
+Config                                              | GCE   | Vagrant | AWS | Azure
+----------------------------------------------------|-------|---------|-----|------
+[cadvisor](cadvisor/)                               | M n   | M n     | M n | M n
+[debian-auto-upgrades](debian-auto-upgrades/)       | M n   | M n     | M n | M n
+[docker](docker/)                                   | M n   | M n     | M n |   n
+[etcd](etcd/)                                       | M     | M       | M   | M
+[fluentd-es](fluentd-es/) (pillar conditional)      | M n   | M n     | M n | M n
+[fluentd-gcp](fluentd-gcp/) (pillar conditional)    | M n   | M n     | M n | M n
+[generate-cert](generate-cert/)                     | M     | M       | M   | M
+[kube-addons](kube-addons/)                         | M     | M       | M   | M
+[kube-apiserver](kube-apiserver/)                   | M     | M       | M   | M
+[kube-controller-manager](kube-controller-manager/) | M     | M       | M   | M
+[kube-proxy](kube-proxy/)                           |   n   |   n     |   n |   n
+[kube-scheduler](kube-scheduler/)                   | M     | M       | M   | M
+[kubelet](kubelet/)                                 | M n   | M n     | M n |   n
+[logrotate](logrotate/)                             | M n   |   n     | M n | M n
+[monit](monit/)                                     | M n   | M n     | M n | M n
+[nginx](nginx/)                                     | M     | M       | M   | M
+[openvpn-client](openvpn-client/)                   |       |         |     |   n
+[openvpn](openvpn/)                                 |       |         |     | M
+[sdn](sdn/) (Vagrant only)                          |   n   | M n     |   n |
+[static-routes](static-routes/) (vsphere only)      |       |         |     |
+[base](base.sls)                                    | M n   | M n     | M n | M n
+[kube-client-tools](kube-client-tools.sls)          | M     | M       | M   | M