Name @beorn7 and @krajorama as maintainers for native histograms

I have been the de-facto maintainer for native histograms from the
beginning. So let's put this into MAINTAINERS.md.

In addition, I hereby proposose George Krajcsovits AKA Krajo as a
co-maintainer. He has contributed a lot of native histogram code, but
more importantly, he has contributed substantially to reviewing other
contributors' native histogram code, up to a point where I was merely
rubberstamping the PRs he had already reviewed. I'm confident that he
is ready to to be granted commit rights as outlined in the
"Maintainers" section of the governance:
https://prometheus.io/governance/#maintainers

According to the same section of the governance, I will announce the
proposed change on the developers mailing list and will give some time
for lazy consensus before merging this PR.

Signed-off-by: beorn7 <beorn@grafana.com>

MAINTAINERS.md

@@ -8,6 +8,8 @@ Julien Pivotto (<roidelapluie@prometheus.io> / @roidelapluie) and Levi Harrison
   * `k8s`: Frederic Branczyk (<fbranczyk@gmail.com> / @brancz)
 * `documentation`
   * `prometheus-mixin`: Matthias Loibl (<mail@matthiasloibl.com> / @metalmatze)
+* `model/histogram` and other code related to native histograms: Björn Rabenstein (<beorn@grafana.com> / @beorn7), 
+George Krajcsovits (<gyorgy.krajcsovits@grafana.com> / @krajorama)
 * `storage`
   * `remote`: Callum Styan (<callumstyan@gmail.com> / @cstyan), Bartłomiej Płotka (<bwplotka@gmail.com> / @bwplotka), Tom Wilkie (<tom.wilkie@gmail.com> / @tomwilkie)
 * `tsdb`: Ganesh Vernekar (<ganesh@grafana.com> / @codesome), Bartłomiej Płotka (<bwplotka@gmail.com> / @bwplotka), Jesús Vázquez (<jesus.vazquez@grafana.com> / @jesusvazquez)
@@ -17,6 +19,7 @@ Julien Pivotto (<roidelapluie@prometheus.io> / @roidelapluie) and Levi Harrison
     * `module`: Augustin Husson (<husson.augustin@gmail.com> @nexucis)
 * `Makefile` and related build configuration: Simon Pasquier (<pasquier.simon@gmail.com> / @simonpasquier), Ben Kochie (<superq@gmail.com> / @SuperQ)
 
+
 For the sake of brevity, not all subtrees are explicitly listed. Due to the
 size of this repository, the natural changes in focus of maintainers over time,
 and nuances of where particular features live, this list will always be