docs: Add readme and changelog for extra body param

Author: JanEbbing

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+* Added feature to add arbitrary body parameters in `translate_text`
 ### Deprecated
 * Dropped support for Python 3.8 and older.
 ### Security

README.md

@@ -173,6 +173,9 @@ arguments are:
     model that minimizes response time, at the cost of translation quality.
 - `tag_handling`: type of tags to parse before translation, options are `'html'`
   and `'xml'`.
+- `extra_body_parameter`: Dictionary of extra parameters to pass in the body of
+  the HTTP request. Mostly used by DeepL employees to test functionality, or for
+  beta programs.
 
 The following options are only used if `tag_handling` is `'xml'`: