add documentation on OnlyOffice upgrade

onlyoffice-docker-upgrade.rst

+Upgrading a Docker-based installation of OnlyOffice
+===================================================
+
+First of all, backup/snapshot OnlyOffice data through a snapshot or as described at https://helpcenter.onlyoffice.com/it/server/docker/community/upgrade-docker.aspx#Step1
+
+Upgrade OnlyOffice installed with docker containers::
+
+  cd /tmp/
+  wget http://download.onlyoffice.com/install/install.sh
+  chmod +x install.sh
+  sudo ./install.sh -u true -os true
+
+Example output::
+
+  5.2.2.2: Pulling from onlyoffice/documentserver
+  3b37166ec614: Already exists 
+  504facff238f: Already exists 
+  ebbcacd28e10: Already exists 
+  c7fb3351ecad: Already exists 
+  2e3debadcbf7: Already exists 
+  044400dde582: Pull complete 
+  90d8e1fc9abb: Pull complete 
+  bc60a37a132e: Pull complete 
+  8b536ff60bda: Pull complete 
+  Digest: sha256:c83f785f747785faa24cc755cdf070caf49dd7b763b1482046b77077ad46070e
+  Status: Downloaded newer image for onlyoffice/documentserver:5.2.2.2
+  Preparing for shutdown, it can take a lot of time, please wait...Done
+  stop container:
+  onlyoffice-document-server
+  remove container:
+  onlyoffice-document-server
+  check removed container: onlyoffice-document-server
+  10f08d8c09b4f0cc6c37183edd93ac5484609b3e1646ed1380254570e5fd5950
+  The latest version of ONLYOFFICE MAIL SERVER is already installed.
+  59004b9bc91bddc566e4256c5ba6d4f280d749bbf2b629f6506b9f90bb2af8ae
+  The latest version of ONLYOFFICE COMMUNITY SERVER is already installed.
+  ad10b35885138a019d0b8e2a865798e7c6824c7ba42e50960b54e40b9712a32f
+  
+  Thank you for installing ONLYOFFICE.
+  In case you have any questions contact us via http://support.onlyoffice.com or visit our forum at http://dev.onlyoffice.org
+