docs cluster-config: Document cluster configuration files

Adds documentation for the YAML files describing cluster configuration.

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+Cluster configuration
+==========================
+
+Every cluster (or "environment") that requires individual configuration is specified in
+a very simple YAML file in Kontemplate.
+
+An example file for a hypothetical test environment could look like this:
+
+```yaml
+---
+context: k8s.test.mydomain.com
+global:
+  clusterName: test-cluster
+  defaultReplicas: 2
+import:
+  - test-secrets.yaml
+include:
+  - name: gateway
+    path: tools/nginx
+    values:
+      tlsDomains:
+        - test.oslo.pub
+        - test.tazj.in
+  - path: backend
+    values:
+      env: test
+    include:
+      - name: blog
+        values:
+          url: test.tazj.in
+      - name: pub-service
+```
+
+## Fields
+
+This is documentation for the individual fields in a cluster context file.
+
+### `context`
+
+The `context` field contains the name of the kubectl-context. You can list context names with
+'kubectl config get-contexts'.
+
+This must be set here so that Kontemplate can use the correct context when calling kubectl.
+
+This field is **required**.
+
+### `global`
+
+The `global` field contains a key/value map of variables that should be available to all resource
+sets in the cluster.
+
+This field is **optional**.
+
+### `import`
+
+The `import` field contains the file names of additional YAML or JSON files from which global
+variables should be loaded. Using this field makes it possible to keep certain configuration that
+is the same for some, but not all, clusters in a common place.
+
+This field is **optional**.
+
+### `include`
+
+The `include` field contains the actual resource sets to be included in the cluster.
+
+Information about the structure of resource sets can be found in the [resource set documentation][].
+
+This field is **required**.
+
+## External variables
+
+As mentioned above, extra variables can be loaded from additional YAML or JSON files. Assuming you
+have a file called `test-secrets.yaml` which contains variables that should be shared between a `test`
+and `dev` cluster, you could include it in your context as such:
+
+```
+# test-secrets.yaml:
+mySecretVar: foo-bar-12345
+
+# test-cluster.yaml:
+context: k8s.test.mydomain.com
+include:
+  - test-secrets.yaml
+
+# dev-cluster.yaml:
+context: k8s.dev.mydomain.com
+include:
+  - test-secrets.yaml
+```
+
+The variable `mySecretVar` is then available as a global variable.
+
+[resource set documentation]: resource-sets.md