release: Volunteering for 2.18 release. (#7046)

👋 

I am happy to do next release. This is because we spent a lot time recent weeks in query optimizations on Prometheus, Thanos, and Cortex, so 2.18 release might have significant changes like:

* Chunk Iterators finally ❤️ 
* @pstibrany and @pracucci major optimizations to postings
* Potentially https://github.com/prometheus/prometheus/issues/6878 with @mkabischev help. 

I know I was a release Shephard just in December, but given that I have full context for those I feel like I can help in releasing 2.18 a lot.

RELEASE.md

@@ -22,7 +22,8 @@ Release cadence of first pre-releases being cut is 6 weeks.
 | v2.15          | 2019-12-18                                 | Bartek Plotka (GitHub: @bwplotka)           |
 | v2.16          | 2020-01-29                                 | Callum Styan (GitHub: @cstyan)              |
 | v2.17          | 2020-03-11                                 | Julien Pivotto (GitHub: @roidelapluie)      |
-| v2.18          | 2020-04-22                                 | **searching for volunteer**                 |
+| v2.18          | 2020-04-22                                 | Bartek Plotka (GitHub: @bwplotka)           |
+| v2.19          | 2020-06-03                                 | **searching for volunteer**                 |
 
 If you are interested in volunteering please create a pull request against the [prometheus/prometheus](https://github.com/prometheus/prometheus) repository and propose yourself for the release series of your choice.