more documentation, 12 more code snippets
diff --git a/doc/snippets/MatrixBase_zero.cpp b/doc/snippets/MatrixBase_zero.cpp
new file mode 100644
index 0000000..8b4af04
--- /dev/null
+++ b/doc/snippets/MatrixBase_zero.cpp
@@ -0,0 +1,2 @@
+cout << Matrix2d::zero() << endl;
+cout << RowVector4i::zero() << endl;