Add a note in example to prefer eval_rule over eval_query (#204)

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

examples/regorus.rs

@@ -70,7 +70,14 @@ fn rego_eval(
         engine.set_input(input);
     }
 
-    // Evaluate query.
+    // Note: The `eval_query` function is used below since it produces output
+    // in the same format as OPA. It also allows evaluating arbitrary statements
+    // as queries.
+    //
+    // Most applications will want to use `eval_rule` instead.
+    // It is faster since it does not have to parse the query string.
+    // It also returns the value of the rule directly and thus is easier
+    // to use.
     let results = engine.eval_query(query, enable_tracing)?;
 
     println!("{}", serde_json::to_string_pretty(&results)?);