add comment to tox (#621)

Author: alrex

tox.ini

@@ -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