SNOW-979340 added description how to enable Arrow (#822)

### Description
Added description how to enable Arrow

### Checklist
- [x] Code compiles correctly
- [x] Code is formatted according to [Coding Conventions](../CodingConventions.md)
- [x] Created tests which fail without the change (if possible)
- [x] All tests passing (`dotnet test`)
- [x] Extended the README / documentation, if necessary
- [x] Provide JIRA issue id (if possible) or GitHub issue id in PR name

README.md

@@ -448,6 +448,30 @@ The .NET driver supports the following mappings from .NET to Snowflake data type
 | `byte` | `BINARY` |
 | `datetime` | `DATE` |
 
+Arrow data format
+-----------------
+
+The .NET connector, starting with v2.1.3, supports the [Arrow data format](https://arrow.apache.org/) 
+as a [preview](https://docs.snowflake.com/en/release-notes/preview-features) feature for data transfers
+between Snowflake and a .NET client. The Arrow data format avoids extra
+conversions between binary and textual representations of the data. The Arrow
+data format can improve performance and reduce memory consumption in clients.
+
+The data format is controlled by the
+DOTNET_QUERY_RESULT_FORMAT parameter. To use Arrow format, execute:
+
+```snowflake
+-- at the session level
+ALTER SESSION SET DOTNET_QUERY_RESULT_FORMAT = ARROW;
+-- or at the user level
+ALTER USER SET DOTNET_QUERY_RESULT_FORMAT = ARROW;
+-- or at the account level
+ALTER ACCOUNT SET DOTNET_QUERY_RESULT_FORMAT = ARROW;
+```
+The valid values for the parameter are:
+
+- ARROW
+- JSON (default)
 
 Run a Query and Read Data
 -------------------------