Document YCM_EP_INSTALL_DIR

[traversaro]

For users that use directly the robotology-superbuild, the fact that the robotology-superbuild install all its packages in <build>/install is typically the desired behaviour and clear. However, for some specific use case (for example installation in a Docker images) it is sometimes desirable to install all the package somewhere else, using the YCM_EP_INSTALL_DIR . At the moment, this variable is document in YCM documentation, but there is too hidden to be found easily by robotology-superbuild users. We should document this workflow, clarifying anyhow that this is only supported for one-shot installation of the superbuild, and not for usual use.

Related issues:

• https://github.com/robotology/robotology-superbuild/pull/316
• https://github.com/robotology/robotology-superbuild/pull/337

[traversaro]

cc @pattacini @Giulero