add performance issues section to doc

Author: espdev

src/lib.rs

@@ -110,6 +110,15 @@
 //! but not a slice of `AsArray` array-like because `ndarray::Array` does not implement `AsRef` trait
 //! currently. In the future we might be able to support `AsArray` in n-dimensional grid data case.
 //!
+//! # Performance Issues
+//!
+//! Currently, the performance of computation of smoothing splines might be very low for a large data.
+//!
+//! The algorithm of sparse matrices mutliplication in sprs crate is not optimized for large diagonal
+//! matrices which causes a poor performance of computation of smoothing splines.
+//! See [issue](https://github.com/vbarrielle/sprs/issues/184) for details.
+//!
+
 
 mod errors;
 mod ndarrayext;