Added XML decoder (#1044)

pkg/yqlib/doc/operators/keys.md

@@ -0,0 +1,114 @@
+# Keys
+
+Use the `keys` operator to return map keys or array indices. 
+
+## Map keys
+Given a sample.yml file of:
+```yaml
+dog: woof
+cat: meow
+```
+then
+```bash
+yq eval 'keys' sample.yml
+```
+will output
+```yaml
+- dog
+- cat
+```
+
+## Array keys
+Given a sample.yml file of:
+```yaml
+- apple
+- banana
+```
+then
+```bash
+yq eval 'keys' sample.yml
+```
+will output
+```yaml
+- 0
+- 1
+```
+
+## Retrieve array key
+Given a sample.yml file of:
+```yaml
+- 1
+- 2
+- 3
+```
+then
+```bash
+yq eval '.[1] | key' sample.yml
+```
+will output
+```yaml
+1
+```
+
+## Retrieve map key
+Given a sample.yml file of:
+```yaml
+a: thing
+```
+then
+```bash
+yq eval '.a | key' sample.yml
+```
+will output
+```yaml
+a
+```
+
+## No key
+Given a sample.yml file of:
+```yaml
+{}
+```
+then
+```bash
+yq eval 'key' sample.yml
+```
+will output
+```yaml
+```
+
+## Update map key
+Given a sample.yml file of:
+```yaml
+a:
+  x: 3
+  y: 4
+```
+then
+```bash
+yq eval '(.a.x | key) = "meow"' sample.yml
+```
+will output
+```yaml
+a:
+  meow: 3
+  y: 4
+```
+
+## Get comment from map key
+Given a sample.yml file of:
+```yaml
+a:
+  # comment on key
+  x: 3
+  y: 4
+```
+then
+```bash
+yq eval '.a.x | key | headComment' sample.yml
+```
+will output
+```yaml
+comment on key
+```
+