add select snippet
diff --git a/doc/snippets/MatrixBase_select.cpp b/doc/snippets/MatrixBase_select.cpp
new file mode 100644
index 0000000..54893be
--- /dev/null
+++ b/doc/snippets/MatrixBase_select.cpp
@@ -0,0 +1,6 @@
+MatrixXi m(3, 3);
+m << 1, 2, 3, 
+     4, 5, 6, 
+     7, 8, 9;
+m = (m.cwise() >= 5).select(-m, m);
+cout << m << endl;