Update changelog

Author: SammyK

CHANGELOG.md

@@ -6,8 +6,10 @@ Starting with version 5, the Facebook PHP SDK follows [SemVer](http://semver.org
 ## 6.x
 
 - 6.0.0 (Sometime in 2017)
-  - Bumped PHP version requirement to 5.6 (#607)
+  - Bumped minimum PHP version requirement to 5.6 (#607, #647)
   - Replace custom CSPRNG implementation with `paragonie/random_compat` (#644)
+  - Removed the built-in autoloader in favor of composer's autoloader (#646)
+  - Big integers in signed requests get decoded as `string` instead of `float` (#699)
 
 ## 5.x