Add comments for compression streams use counters (web-platform-dx#601)

Author: foolip

feature-group-definitions/compression-streams.yml

@@ -1,8 +1,8 @@
 name: Compression streams
 spec: https://wicg.github.io/compression/
 usage_stats:
-  - https://chromestatus.com/metrics/feature/timeline/popularity/3060
-  - https://chromestatus.com/metrics/feature/timeline/popularity/3061
+  - https://chromestatus.com/metrics/feature/timeline/popularity/3060 # CompressionStream
+  - https://chromestatus.com/metrics/feature/timeline/popularity/3061 # DecompressionStream
 compat_features:
   - api.CompressionStream
   - api.CompressionStream.CompressionStream