Add MAINTAINERS file

to document accountability.

Signed-off-by: Matthias Rampke <matthias@prometheus.io>

MAINTAINERS.md

+## Maintainers
+
+* Antoine Leroyer [@aleroyer](https://github.com/aleroyer)
+
+## Sponsor
+
+* Matthias Rampke <matthias@prometheus.io>