Add 5.0.1 release notes and a few unreleased features. (cherry picked from commit 91526464aef156bb0f847db6e00f0b971f9d9ac8)
Eigen is a C++ template library for linear algebra: matrices, vectors, numerical solvers, and related algorithms.
For more information go to http://eigen.tuxfamily.org/ or https://libeigen.gitlab.io/docs/.
For pull request, bug reports, and feature requests, go to https://gitlab.com/libeigen/eigen.