If sin \({\rm{x}} = \frac{1}{{\sqrt 5 }},\) sin \({\rm{y}} = \frac{1}{{\sqrt {10} }}\), where \(0 < {\rm{x}} < \frac{{\rm{\pi }}}{2},{\rm{\;}}0 < {\rm{y}} < \frac{{\rm{\pi }}}{2},\) then what is (x + y) equal to?