Add CHANGELOG entry

Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>

CHANGELOG.md

@@ -31,6 +31,9 @@ We use *breaking* word for marking changes that are not backward compatible (rel
 - [2513](https://github.com/thanos-io/thanos/pull/2513) Tools: Moved `thanos bucket` commands to `thanos tools bucket`, also
 moved `thanos check rules` to `thanos tools rules-check`. `thanos tools rules-check` also takes rules by `--rules` repeated flag not argument
 anymore.
+- [#2596](https://github.com/thanos-io/thanos/pull/2596) Update to Prometheus [@cd73b3d33e064bbd846fc7a26dc8c313d46af382](https://github.com/prometheus/prometheus/commit/cd73b3d33e064bbd846fc7a26dc8c313d46af382) which falls in between v2.17.0 and v2.18.0.
+    - TSDB now supports ssolation of append and queries.
+    - TSDB now holds less WAL files after Head Truncation.
 
 ## [v0.12.2](https://github.com/thanos-io/thanos/releases/tag/v0.12.2) - 2020.04.30