Add information to changelog and README

CHANGELOG.md

@@ -4,6 +4,9 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## 0.12.0
+ - Introduce `Format` trait [#219]
+
 ## 0.11.0 - 2021-03-17
  - The `Config` type got a builder-pattern `with_merged()` method [#166].
  - A `Config::set_once()` function was added, to set an value that can be
@@ -16,6 +19,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 [#172]: https://github.com/mehcode/config-rs/pull/172
 [#169]: https://github.com/mehcode/config-rs/pull/169
 [#175]: https://github.com/mehcode/config-rs/pull/169
+[#219]: https://github.com/mehcode/config-rs/pull/219
 
 ## 0.10.1 - 2019-12-07
  - Allow enums as configuration keys [#119]

README.md

@@ -30,13 +30,23 @@
 config = "0.11"
 ```
 
+### Feature flags
+
  - `ini` - Adds support for reading INI files
  - `json` - Adds support for reading JSON files
  - `yaml` - Adds support for reading YAML files
  - `toml` - Adds support for reading TOML files
  - `ron` - Adds support for reading RON files
  - `json5` - Adds support for reading JSON5 files
 
+### Support for custom formats
+
+Library provides out of the box support for most renowned data formats such as JSON or Yaml. Nonetheless, it contains an extensibility point - a `Format` trait that, once implemented, allows seamless integration with library's APIs using custom, less popular or proprietary data formats.
+
+See [custom_format](https://github.com/mehcode/config-rs/tree/master/examples/custom_format) example for more information.
+
+### More
+
 See the [documentation](https://docs.rs/config) or [examples](https://github.com/mehcode/config-rs/tree/master/examples) for
 more usage information.