Added XML decoder (#1044)

pkg/yqlib/doc/usage/xml.md

@@ -0,0 +1,88 @@
+# XML
+
+At the moment, `yq` only supports decoding `xml` (into one of the other supported output formats).
+
+As yaml does not have the concept of attributes, these are converted to regular fields with a prefix to prevent clobbering. Consecutive xml nodes with the same name are assumed to be arrays.
+
+All values in XML are assumed to be strings - but you can use `from_yaml` to parse them into their correct types:
+
+
+```
+yq e -p=xml '.myNumberField |= from_yaml' my.xml
+```
+
+## Parse xml: simple
+Given a sample.xml file of:
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<cat>meow</cat>
+```
+then
+```bash
+yq e sample.xml
+```
+will output
+```yaml
+cat: meow
+```
+
+## Parse xml: array
+Consecutive nodes with identical xml names are assumed to be arrays.
+
+Given a sample.xml file of:
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<animal>1</animal>
+<animal>2</animal>
+```
+then
+```bash
+yq e sample.xml
+```
+will output
+```yaml
+animal:
+  - "1"
+  - "2"
+```
+
+## Parse xml: attributes
+Attributes are converted to fields, with the attribute prefix.
+
+Given a sample.xml file of:
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<cat legs="4">
+  <legs>7</legs>
+</cat>
+```
+then
+```bash
+yq e sample.xml
+```
+will output
+```yaml
+cat:
+  +legs: "4"
+  legs: "7"
+```
+
+## Parse xml: attributes with content
+Content is added as a field, using the content name
+
+Given a sample.xml file of:
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<cat legs="4">meow</cat>
+```
+then
+```bash
+yq e sample.xml
+```
+will output
+```yaml
+cat:
+  +content: meow
+  +legs: "4"
+```
+