Added cover group platform (#4638)

* Added doc for MultiCover

* Added parameter description

* entity_id_regex

* Removed winter protection

* Changed to 'group' platform

* Changed 'Group Cover' to 'Cover Group'

* ✏️ Small grammar fix

* ⬆️ Updated ha_release.

source/_components/cover.group.markdown

@@ -0,0 +1,43 @@
+---
+layout: page
+title: "Cover Group"
+description: "Instructions how to setup grouped covers in Home Assistant."
+date: 2018-02-11 10:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+ha_category: Cover
+ha_release: 0.66
+ha_iot_class: "Local Push"
+logo: home-assistant.png
+---
+
+The `group` platform can create a cover that combines several cover entities into one.
+
+To enable `Cover Groups` in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+cover:
+  - platform: group
+    entities:
+      - cover.hall_window
+      - cover.living_room_window
+```
+
+{% configuration %}
+  name:
+    description: Name to use in the frontend.
+    required: false
+    type: string
+    default: "Cover Group"
+  entities:
+    description: List of all cover entities you want to control.
+    required: true
+    type: [string | list]
+{% endconfiguration %}
+
+## {% linkable_title Functionality %}
+
+It works best if you group covers with the same supported features together (like support for `open`/`close`/`stop`/`position`/`tilt controls`), but is not limited to it. In case you have bundled covers with different features together, the controls will only affect those covers that support the actions.