Add as maintainer of promtool
Julien Pivotto (<roidelapluie@prometheus.io> / @roidelapluie) is the main/default maintainer, some parts of the codebase have other maintainers:
+* `cmd`
+ * `promtool`: David Leadbeater (<dgl@dgl.cx> / @dgl)
* `discovery`
* `k8s`: Frederic Branczyk (<fbranczyk@gmail.com> / @brancz)
* `documentation`
