Add SOGo HOW TO for changing theme and logo

docs/u_e-sogo.md

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+
+SOGo is used for accessing your mails via a webbrowser, adding and sharing your contacts or calendars. For a more in-depth documentation on SOGo please visit its [own documentation](http://wiki.sogo.nu/).
+
+## Change Theme
+
+You can change SOGo's theme by editing `data/conf/sogo/sogo.conf`. Per default it uses a blue theme, which you can change e.g. to a green or red theme. More colored themes will be supported in the future.
+After you edited said file you need to restart the SOGO container with `docker-compose restart sogo-mailcow` or via the Mailcow UI.
+
+##### Example (`data/conf/sogo/sogo.conf`, line 17):
+```
+before:
+SOGoUIAdditionalJSFiles = (js/theme-blue.js);
+
+after:
+SOGoUIAdditionalJSFiles = (js/theme-red.js);
+```
+
+## Change Logo
+You can change SOGo's logo by replacing `data/Dockerfiles/sogo/sogo-full.svg`.
+After you replaced said file you need to rebuild the container by executing `docker-compose build sogo-mailcow` and start it with `docker-compose up -d sogo-mailcow`.