Added XML decoder (#1044)

pkg/yqlib/doc/operators/has.md

@@ -0,0 +1,70 @@
+# Has
+
+This is operation that returns true if the key exists in a map (or index in an array), false otherwise.
+
+## Has map key
+Given a sample.yml file of:
+```yaml
+- a: yes
+- a: ~
+- a:
+- b: nope
+```
+then
+```bash
+yq eval '.[] | has("a")' sample.yml
+```
+will output
+```yaml
+true
+true
+true
+false
+```
+
+## Select, checking for existence of deep paths
+Simply pipe in parent expressions into `has`
+
+Given a sample.yml file of:
+```yaml
+- a:
+    b:
+      c: cat
+- a:
+    b:
+      d: dog
+```
+then
+```bash
+yq eval '.[] | select(.a.b | has("c"))' sample.yml
+```
+will output
+```yaml
+a:
+  b:
+    c: cat
+```
+
+## Has array index
+Given a sample.yml file of:
+```yaml
+- []
+- [1]
+- [1, 2]
+- [1, null]
+- [1, 2, 3]
+
+```
+then
+```bash
+yq eval '.[] | has(1)' sample.yml
+```
+will output
+```yaml
+false
+false
+true
+true
+true
+```
+