Added XML decoder (#1044)

pkg/yqlib/doc/operators/headers/file-operators.md

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+# File Operators
+
+File operators are most often used with merge when needing to merge specific files together. Note that when doing this, you will need to use `eval-all` to ensure all yaml documents are loaded into memory before performing the merge (as opposed to `eval` which runs the expression once per document).
+
+Note that the `fileIndex` operator has a short alias of `fi`.
+
+## Merging files
+Note the use of eval-all to ensure all documents are loaded into memory.
+```bash
+yq eval-all 'select(fi == 0) * select(filename == "file2.yaml")' file1.yaml file2.yaml
+```