Document Proxy-Wasm ABI v0.2.0. (#51)

Signed-off-by: Piotr Sikora <[email protected]>

Author: PiotrSikora

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+
+## 0.2.1 - 2020-08-04
+
+### Added
+
+- Added `proxy_get_log_level`.