Ensure that the node.js binary is available as nodejs

Debian-like official distributions like Ubuntu 14.04 install the
node.js binary under a different name than provided by upstream
node.js. Document how to set an alias with update-alternatives.

Signed-off-by: Gottfried Hoffmann <gottfried@blenderdiplom.com>
Signed-off-by: Wolfgang Mauerer <wolfgang.mauerer@siemens.com>