Update install.md
Update install.md

Adding Windows default Eigen path.

Adding notice saying CMake will attempt to automatically locate Eigen.
ds283/splinter,bgrimstad/splinter,ds283/splinter,bgrimstad/splinter,bgrimstad/splinter,lasote/multivariate-splines,bgrimstad/splinter,ds283/splinter,ds283/splinter,ds283/splinter,bgrimstad/splinter