Prepare to release 1.6.4

Author: colinodell

CHANGELOG.md

@@ -4,6 +4,8 @@ Updates should follow the [Keep a CHANGELOG](https://keepachangelog.com/) princi
 
 ## [Unreleased][unreleased]
 
+## [1.6.4] - 2021-06-19
+
 ### Changed
 
  - Optimized attribute parsing to avoid inspecting every space character (30% performance boost)
@@ -463,7 +465,8 @@ No changes were made since 1.0.0-rc1.
    - Removed `DelimiterStack::iterateByCharacters()` (use the new `processDelimiters()` method instead)
    - Removed the protected `DelimiterStack::findMatchingOpener()` method
 
-[unreleased]: https://github.com/thephpleague/commonmark/compare/1.6.3...1.6
+[unreleased]: https://github.com/thephpleague/commonmark/compare/1.6.4...1.6
+[1.6.4]: https://github.com/thephpleague/commonmark/compare/1.6.3...1.6.4
 [1.6.3]: https://github.com/thephpleague/commonmark/compare/1.6.2...1.6.3
 [1.6.2]: https://github.com/thephpleague/commonmark/compare/1.6.1...1.6.2
 [1.6.1]: https://github.com/thephpleague/commonmark/compare/1.6.0...1.6.1