Update AUTHORS, CHANGELOG.md for null_safety update (#25)

diff --git a/AUTHORS b/AUTHORS
index d653058..0ba9403 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -6,3 +6,4 @@
 
 Boris Kaul <localvoid@gmail.com>
 Kwang Yul Seo <kwangyul.seo@gmail.com>
+Philip Sieder <philip.sieder@gmx.de>
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8121061..1cd7ca4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
-## 1.0.4-dev
-   * Require Dart 2.2.0.
+## 2.0.0-nullsafety
+   * Upgrade to sound null safety
+   * Require Dart 2.12.0-0
 
 ## 1.0.3
    * Require Dart 2.0.0.