add missing code snippets for newer Matrix methods and PartialLU::solve()
diff --git a/doc/snippets/PartialLU_solve.cpp b/doc/snippets/PartialLU_solve.cpp
new file mode 100644
index 0000000..1244143
--- /dev/null
+++ b/doc/snippets/PartialLU_solve.cpp
@@ -0,0 +1,8 @@
+MatrixXd A = MatrixXd::Random(3,3);
+MatrixXd B = MatrixXd::Random(3,2);
+cout << "Here is the invertible matrix A:" << endl << A << endl;
+cout << "Here is the matrix B:" << endl << B << endl;
+MatrixXd X;
+if(A.lu().solve(B, &X))
+cout << "Here is the (unique) solution X to the equation AX=B:" << endl << X << endl;
+cout << "Relative error: " << (A*X-B).norm() / B.norm() << endl;