Add a proper INSTALL documentation

MANAGE.md

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+
+Upgrading the mail server
+=========================
+
+First check upstream for changes in the ``docker-compose.yml`` or in the
+``freeposte.env`` files. Update these files, then simply pull the latest
+images and recreate the containers :
+
+```
+docker-compose pull
+docker-compose up -d
+```
+
+Monitoring the mail server
+==========================
+
+Logs are managed by Docker directly. You can easily read your logs using :
+
+```
+docker-compose logs
+```
+
+Docker is able to forward logs to multiple log engines. Read the following documentation or details: https://docs.docker.com/engine/admin/logging/overview/.