Add markewaite as a maintainer of throttle-concurrent-builds plugin

Pull request to merge:

* jenkinsci/throttle-concurrent-builds-plugin#339

Current maintainers that can approve the addition:

* @abayer
* @basil
* @oleg-nenashev

Author: MarkEWaite

permissions/plugin-throttle-concurrents.yml

@@ -10,3 +10,4 @@ developers:
   - "abayer"
   - "basil"
   - "oleg_nenashev"
+  - "markewaite"