Add explanation about WIP status to MR form.

![Screen_Shot_2015-05-15_at_13.48.49](https://gitlab.com/gitlab-org/gitlab-ce/uploads/b3f70585255a043f07c61570549a90ba/Screen_Shot_2015-05-15_at_13.48.49.png)

![Screen_Shot_2015-05-15_at_13.49.01](https://gitlab.com/gitlab-org/gitlab-ce/uploads/c9992708c8ef5ef4fa2bcc2c3173b8e3/Screen_Shot_2015-05-15_at_13.49.01.png)

See merge request !666