Add changelog for 300
This MR was automatically created from CI, https://gitlab.com/gitlab-org/gitter/gitter-android-app/-/jobs/169605535
We copied CURRENT_VERSION.txt
-> 300.txt
This MR was automatically created from CI, https://gitlab.com/gitlab-org/gitter/gitter-android-app/-/jobs/169605535
We copied CURRENT_VERSION.txt
-> 300.txt