Add section about otterdog, update section wrt ECA validation, fixes #171

Please take a look into these suggested additions to the handbook. It describes the use of Otterdog, which is a tool to support self-service of GitHub resources via a configuration repository to which project committers have access and can suggest changes that will be applied by EF staff.

Also I updated the section wrt ECA validation with updated screenshots and information about branch protection rules that can be setup to enforce the validation prior to a merge.