Document dyn_cmp_dict (#2624)

Author: tustvold

arrow/README.md

@@ -53,6 +53,7 @@ The `arrow` crate provides the following features which may be enabled in your `
 - `chrono-tz` - support of parsing timezone using [chrono-tz](https://docs.rs/chrono-tz/0.6.0/chrono_tz/)
 - `ffi` - bindings for the Arrow C [C Data Interface](https://arrow.apache.org/docs/format/CDataInterface.html)
 - `pyarrow` - bindings for pyo3 to call arrow-rs from python
+- `dyn_cmp_dict` - enables comparison of dictionary arrays within dyn comparison kernels
 
 ## Arrow Feature Status