Added group_by operator

pkg/yqlib/doc/Encoder and Decoder.md

@@ -1,5 +1,7 @@
 Encode operators will take the piped in object structure and encode it as a string in the desired format. The decode operators do the opposite, they take a formatted string and decode it into the relevant object structure.
 
+Note that you can optionally pass an indent value to the encode functions (see below).
+
 These operators are useful to process yaml documents that have stringified embeded yaml/json/props in them.
 ## Encode value as yaml string
 Indent defaults to 2

pkg/yqlib/doc/Group By.md

@@ -0,0 +1,54 @@
+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
+```
+

pkg/yqlib/doc/headers/Group By.md

@@ -0,0 +1 @@
+This is used to group items in an array by an expression.