update README

Author: japsu

README.md

@@ -78,6 +78,7 @@ Python 3.5+ required. Python 2 is not and will not be supported.
 | `!Exists` | JSONPath expression | `!Exists foo` | Returns `true` if the JSONPath expression returns one or more matches, `false` otherwise. |
 | `!Filter` | `test`, `over` | See `tests/test_cond.py` | Takes in a list and only returns elements that pass a predicate. |
 | `!Format` | Format string | `!Format "{foo} {bar!d}"` | Interpolate strings using [Python format strings](https://docs.python.org/3/library/string.html#formatstrings). <br> JSONPath supported in variable lookup (eg. `{people[0].first_name}` will do the right thing). <br> **NOTE:** When the format string starts with `{`, you need to quote it in order to avoid being interpreted as a YAML object. |
+| `!Group` | Accepts the same arguments as `!Loop`, except `template` is optional (default identity), plus the following: <br> `by`: (required) An expression used to determine the key for the current value <br> `result_as`: (optional, string) When evaluating `by`, the enriched `template` is available under this name. | TBD | Makes a dict out of a list. Keys are determined by `by`. Items with the same key are grouped in a list. |
 | `!If` | `test`, `then`, `else` | See `tests/test_cond.py` | Returns one of two values based on a condition. |
 | `!Include` | Path to a template to include | `!Include ../foo.yml` | Renders the requested template at this location. Both absolute and relative paths work. |
 | `!IncludeBase64` | Path to a binary file | `!IncludeBase64 ../foo.pdf` | Loads the given binary file and returns the contents encoded as Base64. |