Added changelog for version 1.20.0. [skip ci]

Author: jlesage

appdefs.xml

@@ -109,6 +109,11 @@ Where:
     </documentation>
     <!-- Changelog of the application. -->
     <history>
+      <release>
+        <version>1.20.0</version>
+        <date>2021-08-28</date>
+        <change>Updated Nginx Proxy Manager to version 2.9.8.</change>
+      </release>
       <release>
         <version>1.19.0</version>
         <date>2021-08-08</date>