docs resource-sets: Document resource set structure

Documents the structure of resource sets and the fields necessary for including
them in cluster configurations.

Also adds some words about nested resource sets and the like.

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+Resource Sets
+================
+
+Resource sets are collections of Kubernetes resources that should be passed to `kubectl` together.
+
+Technically a resource set is simply a folder with a few YAML and/or JSON templates in it.
+
+
+# Creating resource sets
+
+Simply create a folder in your Kontemplate repository and place a YAML or JSON file in it. These
+files get interpreted as [templates][] during Kontemplate runs and variables (as well as template
+logic or functions) will be interpolated.
+
+Refer to the template documentation for information on how to write templates.
+
+## Default variables
+
+Sometimes it is useful to specify default values for variables that should be interpolated during
+a run if the [cluster configuration][] does not specify a variable explicitly.
+
+This can be done simply by placing a `default.yaml` or `default.json` file in the resource set
+folder and filling it with key/value pairs of the intended default variables.
+
+Kontemplate will error during interpolation if any variables are left unspecified.
+
+# Including resource sets
+
+Under the cluster configuration `include` key resource sets are included and required variables
+are specified. For example:
+
+```yaml
+include:
+  - name: some-api
+    values:
+      version: 1.2-SNAPSHOT
+```
+
+This will include a resource set from a folder called `some-api` and set the specified `version` variable.
+
+## Fields
+
+The available fields when including a resource set are these:
+
+### `name`
+
+The `name` field contains the name of the resource set. This name can be used to refer to the resource set
+when specifying explicit includes or excludes during a run.
+
+By default it is assumed that the `name` is the path to the resource set folder, but this can be overridden.
+
+This field is **required**.
+
+### `path`
+
+The `path` field specifies an explicit path to a resource set folder in the case that it should differ from
+the resource set's `name`.
+
+This field is **optional**.
+
+### `values`
+
+The `values` field specifies key/values pairs of variables that should be available during templating.
+
+This field is **optional**.
+
+### `include`
+
+The `include` field specifies additional resource sets that should be included and that should inherit the
+variables of this resource set.
+
+The fully qualified names of "nested" resource sets are set to `${PARENT_NAME}/${CHILD_NAME}` and paths are
+merged in the same way.
+
+This makes it easy to organise different resource sets as "groups" to include / exclude them collectively
+during runs.
+
+This field is **optional**.
+
+## Multiple includes
+
+Resource sets can be included multiple times with different configurations. In this case it is recommended
+to set the `path` and `name` fields explicitly. For example:
+
+```yaml
+include:
+  - name: forwarder-europe
+    path: tools/forwarder
+    values:
+      source: europe
+  - name: forwarder-asia
+    path: tools/forwarder
+    values:
+      source: asia
+```
+
+The two different configurations can be referred to by their set names, but will use the same resource
+templates with different configurations.
+
+## Nesting resource sets
+
+As mentioned above for the `include` field, resource sets can be nested. This lets users group resource
+sets in logical ways using simple folder structures.
+
+Assuming a folder structure like:
+
+```
+├── backend
+│   ├── auth-api
+│   ├── message-api
+│   └── order-api
+└── frontend
+    ├── app-page
+    └── login-page
+```
+
+With each of these folders being a resource set, they could be included in a cluster configuration like so:
+
+```yaml
+include:
+  - name: backend
+    include:
+      - name: auth-api
+      - name: message-api
+      - name: order-api
+  - name: frontend:
+    include:
+      - name: app-page
+      - name: login-page
+```
+
+Kontemplate could then be run with, for example, `--include backend` to only include the resource sets nested
+in the backend group. Specific resource sets can also be targeted, for example as `--include backend/order-api`.
+
+Variables specified in the parent resource set are inherited by the children.
+
+### Caveats
+
+Two caveats apply that users should be aware of:
+
+1. The parent resource set can not contain any resource templates itself.
+
+2. Only one level of nesting is supported. Specifying `include` again on a nested resource set will be ignored.
+
+[templates]: templates.md
+[cluster configuration]: cluster-config.md