Added XML decoder (#1044)

pkg/yqlib/doc/operators/document-index.md

@@ -0,0 +1,91 @@
+# Document Index
+
+Use the `documentIndex` operator (or the `di` shorthand) to select nodes of a particular document.
+
+## Retrieve a document index
+Given a sample.yml file of:
+```yaml
+a: cat
+---
+a: frog
+```
+then
+```bash
+yq eval '.a | documentIndex' sample.yml
+```
+will output
+```yaml
+0
+---
+1
+```
+
+## Retrieve a document index, shorthand
+Given a sample.yml file of:
+```yaml
+a: cat
+---
+a: frog
+```
+then
+```bash
+yq eval '.a | di' sample.yml
+```
+will output
+```yaml
+0
+---
+1
+```
+
+## Filter by document index
+Given a sample.yml file of:
+```yaml
+a: cat
+---
+a: frog
+```
+then
+```bash
+yq eval 'select(documentIndex == 1)' sample.yml
+```
+will output
+```yaml
+a: frog
+```
+
+## Filter by document index shorthand
+Given a sample.yml file of:
+```yaml
+a: cat
+---
+a: frog
+```
+then
+```bash
+yq eval 'select(di == 1)' sample.yml
+```
+will output
+```yaml
+a: frog
+```
+
+## Print Document Index with matches
+Given a sample.yml file of:
+```yaml
+a: cat
+---
+a: frog
+```
+then
+```bash
+yq eval '.a | ({"match": ., "doc": documentIndex})' sample.yml
+```
+will output
+```yaml
+match: cat
+doc: 0
+match: frog
+doc: 1
+```
+