Describe and hoist stripped state to a first-class citizen (#3606)

* Describe and hoist stripped state to a first-class citizen

Fixes https://github.com/matrix-org/matrix-doc/issues/3413
MSC: https://github.com/matrix-org/matrix-doc/pull/3173

* Add changelog

* may->can for clarity

* Update text per review

data/event-schemas/schema/core-event-schema/stripped_state.yaml

@@ -0,0 +1,44 @@
+# Copyright 2019 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Note: this, and the example, are in the `event-schemas` directory because
+# the CS API uses a symlink. In order for the `m.room.member` event to
+# reference this, we'd need to use relative pathing. The symlink makes this
+# difficult because the schema would be at two different locations, with
+# different relative pathing.
+
+title: StrippedStateEvent
+type: object
+description: |-
+  A stripped down state event, with only the `type`, `state_key`,
+  `sender`, and `content` keys.
+properties:
+  content:
+    description: The `content` for the event.
+    title: EventContent
+    type: object
+  state_key:
+    description: The `state_key` for the event.
+    type: string
+  type:
+    description: The `type` for the event.
+    type: string
+  sender:
+    description: The `sender` for the event.
+    type: string
+required:
+  - type
+  - state_key
+  - content
+  - sender