Added pipe and length docs, fix pipe precedence

pkg/yqlib/operator_pipe.go

@@ -0,0 +1,11 @@
+package yqlib
+
+import "container/list"
+
+func PipeOperator(d *dataTreeNavigator, matchingNodes *list.List, pathNode *PathTreeNode) (*list.List, error) {
+	lhs, err := d.GetMatchingNodes(matchingNodes, pathNode.Lhs)
+	if err != nil {
+		return nil, err
+	}
+	return d.GetMatchingNodes(lhs, pathNode.Rhs)
+}