Added XML decoder (#1044)

pkg/yqlib/doc/operators/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
+```
+
+## Get filename
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+then
+```bash
+yq eval 'filename' sample.yml
+```
+will output
+```yaml
+sample.yml
+```
+
+## Get file index
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+then
+```bash
+yq eval 'fileIndex' sample.yml
+```
+will output
+```yaml
+0
+```
+
+## Get file indices of multiple documents
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+And another sample another.yml file of:
+```yaml
+a: cat
+```
+then
+```bash
+yq eval-all 'fileIndex' sample.yml another.yml
+```
+will output
+```yaml
+0
+---
+1
+```
+
+## Get file index alias
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+then
+```bash
+yq eval 'fi' sample.yml
+```
+will output
+```yaml
+0
+```
+