doc(Governance): add @SamuelSchlesinger to reviewers (#608)

Adds @SamuelSchlesinger to the GOVERNANCE file as a reviewer.

Author: fmontesi

GOVERNANCE.md

@@ -46,5 +46,6 @@ Area maintainers are trusted contributors who take ownership of specific areas o
 Reviewers are trusted contributors who provide regular reviewing and technical guidance to PRs to CSLib.
 
 - Ching-Tsun Chou (@ctchou).
-- Thomas Waring (@thomaskwaring). 
+- Samuel Schlesinger (@SamuelSchlesinger).
+- Thomas Waring (@thomaskwaring).
 - Eric Wieser (@eric-wieser), Google DeepMind.