[DOC] Add a little documentation about Alembic

alembic/README

-Generic single-database configuration.
+Alembic must be installed in the virtualenv in order to use right python paths,
\ No newline at end of file
+so it's installed with pip. Commands described in this little documentation
+must be executed from gargantext root directory, ie. /srv/gargantext.
+
+Keep in mind that Alembic only handles SQLAlchemy models: tables created from
+Django ORM must be put out of Alembic sight. See [alembic:exclude] section in
+alembic.ini.
+
+UPGRADE TO LATEST DATABASE VERSION
+
+    alembic upgrade head
+
+
+GENERATE A NEW REVISION
+
+    alembic revision -m "Message for this migration"
+
+    # A migration script is then created in alembic/versions directory. For
+    # example alembic/versions/3adcc9a56557_message_for_this_migration.py
+    # where 3adcc9a56557 is the revision id generated by Alembic.
+    #
+    # This script must be edited to write the migration itself, mainly
+    # in `upgrade` and `downgrade` functions. See Alembic documentation for
+    # further details.
+
+
+GENERATE A REVISION FROM CURRENT STATE
+
+    alembic revision --autogenerate -m "Message for this migration"
+
+    # Alembic should generate a script reflecting changes already made in
+    # database. However it is always a good idea to check it and edit it
+    # manually, Alembic is not always accurate and can't see all alterations.
+    # It should work with basic changes such as model or column creation.