README.md in trackler-2.2.1.104

- old
+ new

@@ -13,9 +13,24 @@
 
 The names must be random: they should not follow a predictable sequence.
 Random names means a risk of collisions. Your solution must ensure that
 every existing robot has a unique name.
 
+### Bonus exercise
+
+Once you get `go test` passing, try `go test -tags bonus`.  This uses a *build
+tag* to enable a test that wasn't previously enabled. Build tags control which
+files should be included in the package. You can read more about those at [the
+Go documentation](https://golang.org/pkg/go/build/#hdr-Build_Constraints).
+
+For this exercise it limits `go test` to only build the tests in the
+`robot_name_test.go` file. We can then include the bonus test in the
+`bonus_test.go` file with the tags flag as described above.
+
+To get the bonus test to pass you'll have to ensure a robot's name has not been
+used before.
+
+
 ## Running the tests
 
 To run the tests run the command `go test` from within the exercise directory.
 
 If the test suite contains benchmarks, you can run these with the `-bench`