docs: add badge to readme (#727)

Signed-off-by: Vitor Bandeira <[email protected]>

Signed-off-by: Vitor Bandeira <[email protected]>

Author: vvbandeira

README.md

@@ -1,6 +1,7 @@
 # OpenROAD Flow
 
 [![Build Status](https://jenkins.openroad.tools/buildStatus/icon?job=OpenROAD-flow-scripts-Public%2Fpublic_tests_all%2Fmaster)](https://jenkins.openroad.tools/view/Public/job/OpenROAD-flow-scripts-Public/job/public_tests_all/job/master/)
+[![Docs](https://readthedocs.org/projects/openroad-flow-scripts/badge/?version=latest)](https://openroad-flow-scripts.readthedocs.io/en/latest/?badge=latest)
 
 OpenROAD Flow is a full RTL-to-GDS flow built entirely on open-source tools.
 The project aims for automated, no-human-in-the-loop digital circuit design