Update indicator description

Author: nicolaskruchten

src/traces/indicator/index.js

@@ -24,7 +24,11 @@ module.exports = {
 
     meta: {
         description: [
-            'TODO: add description'
+            'An indicator is used to visualize single a `value` along with some',
+            'contextual information such as `steps` or a `threshold`, using a',
+            'combination of three visual elements: a number, a delta, and/or a gauge.',
+            'Deltas are taken with respect to a `reference`.',
+            'Gauges can be either angular or bullet (aka linear) gauges.'
         ].join(' ')
     }
 };