Add recommendation to use RESULT-SPACE

This is a suggested resolution for the discussion in https://github.com/ros-infrastructure/rep/issues/78#issuecomment-46446157

Possible alternative `<result-space>` is less preferable IMO due to the special meaning of the `<` and `>` characters on the shell (consider people blindly copy&pasting such instructions to their shell).
Merge pull request #80 from NikolausDemmel/patch-1

Add recommendation to use RESULT-SPACE