Adds extensions to groups (#202)

* Adds extensions to groups

* Remove suggestion about vendor-specific inheritence

technical-reports/format/groups.md

@@ -138,12 +138,6 @@ Suggested ways tools MAY use this property are:
 - A GUI tool that lets users browse or select tokens could display this info alongside the corresponding group or as a tooltip
 - Translation tools could output this as a source code comment
 
-<div class="issue" data-number="72">
-
-Groups may support additional properties like type and description. Should other properties be supported at the group level?
-
-</div>
-
 ### Type
 
 Groups MAY include an optional `$type` property so a type property does not need to be manually added to every token. [See supported "Types"](#types) for more information.
@@ -172,6 +166,46 @@ For example:
 
 </aside>
 
+### Extensions
+
+Groups MAY include an optional `$extensions` property where tools MAY add proprietary, user-, team- or vendor-specific data to a group. When doing so, each tool MUST use a vendor-specific key whose value MAY be any valid JSON data.
+
+Group extensions follow the same rules as [design token extensions](#extensions), the only difference is that they relate to the group itself.
+
+- The keys SHOULD be chosen such that they avoid the likelihood of a naming clash with another vendor's data. The [reverse domain name notation](https://en.wikipedia.org/wiki/Reverse_domain_name_notation) is recommended for this purpose.
+- Tools that process design token files MUST preserve any extension data they do not themselves understand. For example, if a group contains extension data from tool A and the file containing that data is opened by tool B, then tool B MUST include the original tool A extension data whenever it saves a new design token file containing that group.
+
+Note that, since a group's `$extensions` only relate to that group, they do not have any effect on nested groups or tokens.
+
+<aside class="example">
+
+```json
+{
+  "brand": {
+    "$extensions": {
+      "org.example.tool-a": 42,
+      "org.example.tool-b": {
+        "turn-up-to-11": true
+      }
+    },
+    "color": {
+      "acid green": {
+        "$value": "#00ff66",
+        "$type": "color"
+      },
+      "hot pink": {
+        "$value": "#dd22cc",
+        "$type": "color"
+      }
+    }
+  }
+}
+```
+
+In this example, the "brand" group has 2 extensions: `org.example.tool-a` and `org.example.tool-b`.
+
+</aside>
+
 ## Use-cases
 
 ### File authoring & organization