doc: Add a note that datafusion may need to be downgraded after installing it (apache#1348)

`cargo add ballista datafusion tokio` installs the latest versions of
the listed dependencies and ofter Ballista is a bit behind DataFusion,
so the user needs to downgrade it so they match

Author: martin-g

ballista/client/README.md

@@ -117,6 +117,8 @@ To build a simple ballista example, run the following command to add the depende
 cargo add ballista datafusion tokio
 ```
 
+**Important**: Make sure the version of `datafusion` is the same as `ballista`'s! You may need to use a bit older version of `datafusion`!
+
 ```rust,no_run
 use ballista::prelude::*;
 use datafusion::common::Result;