message: Add filesystem documentation
diff: //!
  //! See the source of the [`files` example](https://github.com/ggez/ggez/blob/master/examples/files.rs) for more details.
  //!
  +//! The names of `resources/` and `resources.zip` can be changed with the methods
  +//! [`change_resources_dir_name`](../struct.ContextBuilder.html#method.change_resources_dir_name)
  +//! and
  +//! [`change_resources_zip_name`](../struct.ContextBuilder.html#method.change_resources_zip_name)
  +//! of ContextBuilder.
  +//!
  //! Note that the file lookups WILL follow symlinks! This module's
  //! directory isolation is intended for convenience, not security, so
  //! don't assume it will be secure.
  