update readme [skip-ci] (#354)

Author: mensfeld

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Rdkafka Changelog
 
 ## 0.15.0 (Unreleased)
+- **[Feature]** Add `Admin#delete_group` utility (piotaixr)
 - **[Feature]** Add Create and Delete ACL Feature To Admin Functions (vgnanasekaran)
 - [Enhancement] Bump librdkafka to 2.3.0 (mensfeld)
 - [Enhancement] Increase the `#lag` and `#query_watermark_offsets` default timeouts from 100ms to 1000ms. This will compensate for network glitches and remote clusters operations (mensfeld)