Added XML decoder (#1044)

pkg/yqlib/doc/operators/length.md

@@ -0,0 +1,70 @@
+# Length
+
+Returns the lengths of the nodes. Length is defined according to the type of the node.
+
+## String length
+returns length of string
+
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+then
+```bash
+yq eval '.a | length' sample.yml
+```
+will output
+```yaml
+3
+```
+
+## null length
+Given a sample.yml file of:
+```yaml
+a: null
+```
+then
+```bash
+yq eval '.a | length' sample.yml
+```
+will output
+```yaml
+0
+```
+
+## Map length
+returns number of entries
+
+Given a sample.yml file of:
+```yaml
+a: cat
+c: dog
+```
+then
+```bash
+yq eval 'length' sample.yml
+```
+will output
+```yaml
+2
+```
+
+## Array length
+returns number of elements
+
+Given a sample.yml file of:
+```yaml
+- 2
+- 4
+- 6
+- 8
+```
+then
+```bash
+yq eval 'length' sample.yml
+```
+will output
+```yaml
+4
+```
+