Update mongo license.md to note SSPL

Closes https://github.com/docker-library/mongo/issues/324

mongo/license.md

@@ -1 +1,3 @@
-View [license information](https://github.com/mongodb/mongo/blob/7c3cfac300cfcca4f73f1c3b18457f0f8fae3f69/README#L71) for the software contained in this image.
+View [license information](https://github.com/mongodb/mongo/blob/6ea81c883e7297be99884185c908c7ece385caf8/README#L89-L95) for the software contained in this image.
+
+It is relevant to note the change from AGPL to SSPLv1 for all versions after October 16, 2018.