Added XML decoder (#1044)

pkg/yqlib/doc/operators/headers/Main.md

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+# NAME
+  *yq* is a portable command-line YAML processor
+
+# SYNOPSIS 
+
+yq [eval/eval-all] [expression] files..
+
+eval/e  - Apply the expression to each document in each yaml file in sequence
+
+eval-all/ea - Loads all yaml documents of all yaml files and runs expression once
+
+# DESCRIPTION
+
+a lightweight and portable command-line YAML processor. `yq` uses [jq](https://github.com/stedolan/jq) like syntax but works with yaml files as well as json. It doesn't yet support everything `jq` does - but it does support the most common operations and functions, and more is being added continuously.
+
+This documentation is also available at https://mikefarah.gitbook.io/yq/
+# QUICK GUIDE 
+
+## Read a value:
+```bash
+yq e '.a.b[0].c' file.yaml
+```
+
+## Pipe from STDIN:
+```bash
+cat file.yaml | yq e '.a.b[0].c' -
+```
+
+## Update a yaml file, inplace
+```bash
+yq e -i '.a.b[0].c = "cool"' file.yaml
+```
+
+## Update using environment variables
+```bash
+NAME=mike yq e -i '.a.b[0].c = strenv(NAME)' file.yaml
+```
+
+## Merge multiple files
+```
+yq ea '. as $item ireduce ({}; . * $item )' path/to/*.yml
+```
+
+## Multiple updates to a yaml file
+```bash
+yq e -i '
+  .a.b[0].c = "cool" |
+  .x.y.z = "foobar" |
+  .person.name = strenv(NAME)
+' file.yaml
+```
+
+See the [documentation](https://mikefarah.gitbook.io/yq/) for more.
+
+# KNOWN ISSUES / MISSING FEATURES
+- `yq` attempts to preserve comment positions and whitespace as much as possible, but it does not handle all scenarios (see https://github.com/go-yaml/yaml/tree/v3 for details)
+- Powershell has its own...opinions: https://mikefarah.gitbook.io/yq/usage/tips-and-tricks#quotes-in-windows-powershell
+
+# BUGS / ISSUES / FEATURE REQUESTS
+
+Please visit the GitHub page https://github.com/mikefarah/yq/.
+