use saamwerk bookdown

Author: lorenzwalthert

CONTRIBUTING.md

@@ -1,4 +1,2 @@
-# Issue Management
-
-This project follows the issue management recommendations outlined by [saamwerk](https://github.com/lorenzwalthert/saamwerk/blob/master/issue-management/labelling-strategy.md).
+This project follows the contributing recommendations outlined by [saamwerk](https://lorenzwalthert.github.io/saamwerk/).
 In particular, issues labelled with `Status: Postponed` are closed even if they are not resolved.