Added XML decoder (#1044)

pkg/yqlib/doc/operators/variable-operators.md

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+# Variable Operators
+
+Like the `jq` equivalents, variables are sometimes required for the more complex expressions (or swapping values between fields).
+
+Note that there is also an additional `ref` operator that holds a reference (instead of a copy) of the path, allowing you to make multiple changes to the same path.
+
+## Single value variable
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+then
+```bash
+yq eval '.a as $foo | $foo' sample.yml
+```
+will output
+```yaml
+cat
+```
+
+## Multi value variable
+Given a sample.yml file of:
+```yaml
+- cat
+- dog
+```
+then
+```bash
+yq eval '.[] as $foo | $foo' sample.yml
+```
+will output
+```yaml
+cat
+dog
+```
+
+## Using variables as a lookup
+Example taken from [jq](https://stedolan.github.io/jq/manual/#Variable/SymbolicBindingOperator:...as$identifier|...)
+
+Given a sample.yml file of:
+```yaml
+"posts":
+  - "title": Frist psot
+    "author": anon
+  - "title": A well-written article
+    "author": person1
+"realnames":
+  "anon": Anonymous Coward
+  "person1": Person McPherson
+```
+then
+```bash
+yq eval '.realnames as $names | .posts[] | {"title":.title, "author": $names[.author]}' sample.yml
+```
+will output
+```yaml
+title: Frist psot
+author: Anonymous Coward
+title: A well-written article
+author: Person McPherson
+```
+
+## Using variables to swap values
+Given a sample.yml file of:
+```yaml
+a: a_value
+b: b_value
+```
+then
+```bash
+yq eval '.a as $x  | .b as $y | .b = $x | .a = $y' sample.yml
+```
+will output
+```yaml
+a: b_value
+b: a_value
+```
+
+## Use ref to reference a path repeatedly
+Note: You may find the `with` operator more useful.
+
+Given a sample.yml file of:
+```yaml
+a:
+  b: thing
+  c: something
+```
+then
+```bash
+yq eval '.a.b ref $x | $x = "new" | $x style="double"' sample.yml
+```
+will output
+```yaml
+a:
+  b: "new"
+  c: something
+```
+