add comment to tox
@@ -171,6 +171,7 @@ deps =
elasticsearch5: elasticsearch>=5.0,<6.0
elasticsearch6: elasticsearch-dsl>=6.0,<7.0
elasticsearch6: elasticsearch>=6.0,<7.0
+ ; FIXME: Elasticsearch >=7 causes CI workflow tests to hang, see open-telemetry/opentelemetry-python-contrib#620
; elasticsearch7: elasticsearch-dsl>=7.0,<8.0
; elasticsearch7: elasticsearch>=7.0,<8.0
sqlalchemy11: sqlalchemy>=1.1,<1.2
