Update README and CHANGELOG for new release

Author: hypest

CHANGELOG.md

@@ -1,4 +1,8 @@
 # Changelog
+## [v1.3.21](https://github.com/wordpress-mobile/AztecEditor-Android/releases/tag/v1.3.21)
+### Changed
+- Some performance improvements, mostly visible via the WPAndroid app https://github.com/wordpress-mobile/gutenberg-mobile/issues/672
+
 ## [v1.3.20](https://github.com/wordpress-mobile/AztecEditor-Android/releases/tag/v1.3.20)
 ### Changed
 - Support for `<em>` for italic and using it as default via the formatting toolbar #777

README.md

@@ -105,7 +105,7 @@ repositories {
 ```
 ```gradle
 dependencies {
-    api ('com.github.wordpress-mobile.WordPress-Aztec-Android:aztec:v1.3.20')
+    api ('com.github.wordpress-mobile.WordPress-Aztec-Android:aztec:v1.3.21')
 }
 ```