added group-by doc

operators/group-by.md

@@ -0,0 +1,56 @@
+# Group By
+
+This is used to group items in an array by an expression.
+
+## Group by field
+Given a sample.yml file of:
+```yaml
+- foo: 1
+  bar: 10
+- foo: 3
+  bar: 100
+- foo: 1
+  bar: 1
+```
+then
+```bash
+yq eval 'group_by(.foo)' sample.yml
+```
+will output
+```yaml
+- - foo: 1
+    bar: 10
+  - foo: 1
+    bar: 1
+- - foo: 3
+    bar: 100
+```
+
+## Group by field, with nuls
+Given a sample.yml file of:
+```yaml
+- cat: dog
+- foo: 1
+  bar: 10
+- foo: 3
+  bar: 100
+- no: foo for you
+- foo: 1
+  bar: 1
+```
+then
+```bash
+yq eval 'group_by(.foo)' sample.yml
+```
+will output
+```yaml
+- - cat: dog
+  - no: foo for you
+- - foo: 1
+    bar: 10
+  - foo: 1
+    bar: 1
+- - foo: 3
+    bar: 100
+```
+