Adding shebang documentation

pkg/yqlib/doc/usage/formatting-expressions.md

@@ -35,6 +35,40 @@ a:
   c: frog
 ```
 
+## Flags in expression files
+You can specify flags on the shebang line, this only works when executing the file directly.
+
+Given a sample.yaml file of:
+```yaml
+a:
+  b: old
+```
+And an 'update.yq' expression file of:
+```bash
+#! yq -oj
+
+# This is a yq expression that updates the map
+# for several great reasons outlined here.
+
+.a.b = "new" # line comment here
+| .a.c = "frog"
+
+# Now good things will happen.
+```
+then
+```bash
+./update.yq sample.yaml
+```
+will output
+```yaml
+{
+  "a": {
+    "b": "new",
+    "c": "frog"
+  }
+}
+```
+
 ## Commenting out yq expressions
 Note that `c` is no longer set to 'frog'. In this example we're calling yq directly and passing the expression file into `--from-file`, this is no different from executing the expression file directly.