Added XML decoder (#1044)

pkg/yqlib/doc/operators/pipe.md

@@ -0,0 +1,37 @@
+# Pipe
+
+Pipe the results of an expression into another. Like the bash operator.
+
+## Simple Pipe
+Given a sample.yml file of:
+```yaml
+a:
+  b: cat
+```
+then
+```bash
+yq eval '.a | .b' sample.yml
+```
+will output
+```yaml
+cat
+```
+
+## Multiple updates
+Given a sample.yml file of:
+```yaml
+a: cow
+b: sheep
+c: same
+```
+then
+```bash
+yq eval '.a = "cat" | .b = "dog"' sample.yml
+```
+will output
+```yaml
+a: cat
+b: dog
+c: same
+```
+