parquet: Add snap option to README (#2847)

I tried using parquet with `default-features = false` (and with some options explicitly enabled), but it didn't support snappy compression. Turns out there is an undocumented option for it.

parquet/README.md

@@ -47,6 +47,7 @@ The `parquet` crate provides the following features which may be enabled in your
 - `flate2` (default) - support for parquet using `gzip` compression
 - `lz4` (default) - support for parquet using `lz4` compression
 - `zstd` (default) - support for parquet using `zstd` compression
+- `snap` (default) - support for parquet using `snappy` compression
 - `cli` - parquet [CLI tools](https://github.com/apache/arrow-rs/tree/master/parquet/src/bin)
 - `experimental` - Experimental APIs which may change, even between minor releases