Create README.md

Author: XuhuaHuang

Eigen/README.md

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+# `Eigen/Dense`
+
+```cpp
+#include <algorithm>
+#include <random>
+// ... other Eigen includes
+VectorXd u{12};
+std::mt19937 rng{std::random_device{}()};
+std::uniform_real_distribution<> dist{-1, 1};
+std::generate(u.data(), u.data() + u.size(), [&]() { return dist(rng); });
+std::student_t_distribution<> tdist{5};
+std::generate(u.data(), u.data() + u.size(), [&]() { return tdist(rng); });
+auto u2 = u.array().abs();
+std::cout << u2.transpose() << std::endl;
+auto max_u = std::max_element(u2.begin(), u2.end() + u2.size());
+std::cout << "Max: " << *max_u << std::endl;
+u2 = u2.unaryExpr([](double x) { return x * x; });
+// std::experimental::linalg::matrix_vector_product
+// std::experimental::linalg::dot
+```