in the README, show usage as a dev-dependency

Fixes #98

Author: asomers

README.md

@@ -35,19 +35,25 @@ mockall = "0.6.0"
 Then use it like this:
 
 ```rust
-use mockall::*;
-use mockall::predicate::*;
-#[automock]
+#[cfg(test)]
+use mockall::{automock, mock, predicate::*};
+#[cfg_attr(test, automock)]
 trait MyTrait {
     fn foo(&self, x: u32) -> u32;
 }
 
-let mut mock = MockMyTrait::new();
-mock.expect_foo()
-    .with(eq(4))
-    .times(1)
-    .returning(|x| x + 1);
-assert_eq!(5, mock.foo(4));
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn mytest() {
+        let mut mock = MockMyTrait::new();
+        mock.expect_foo()
+            .with(eq(4))
+            .times(1)
+            .returning(|x| x + 1);
+        assert_eq!(5, mock.foo(4));
+    }
+}
 ```
 
 See the [API docs](https://docs.rs/mockall) for more information.