Added flatten doc

SUMMARY.md

@@ -28,6 +28,7 @@
   * [Env Variable Operators](operators/env-variable-operators.md)
   * [Equals](operators/equals.md)
   * [File Operators](operators/file-operators.md)
+  * [Flatten](operators/flatten.md)
   * [Group By](operators/group-by.md)
   * [Has](operators/has.md)
   * [Keys](operators/keys.md)

operators/flatten.md

@@ -0,0 +1,71 @@
+# Flatten
+This recursively flattens arrays.
+
+## Flatten
+Recursively flattens all arrays
+
+Given a sample.yml file of:
+```yaml
+- 1
+- - 2
+- - - 3
+```
+then
+```bash
+yq eval 'flatten' sample.yml
+```
+will output
+```yaml
+- 1
+- 2
+- 3
+```
+
+## Flatten with depth of one
+Given a sample.yml file of:
+```yaml
+- 1
+- - 2
+- - - 3
+```
+then
+```bash
+yq eval 'flatten(1)' sample.yml
+```
+will output
+```yaml
+- 1
+- 2
+- - 3
+```
+
+## Flatten empty array
+Given a sample.yml file of:
+```yaml
+- []
+```
+then
+```bash
+yq eval 'flatten' sample.yml
+```
+will output
+```yaml
+[]
+```
+
+## Flatten array of objects
+Given a sample.yml file of:
+```yaml
+- foo: bar
+- - foo: baz
+```
+then
+```bash
+yq eval 'flatten' sample.yml
+```
+will output
+```yaml
+- foo: bar
+- foo: baz
+```
+