Enumerates the migration commands

doc/migration.md

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+# Performing migrations #
+
+## Introduction ##
+
+Migrations on `Tracim` lays on [`gearbox migrate`](http://turbogears.readthedocs.io/en/tg2.3.7/turbogears/migrations.html), which in turn lays on [`alembic`](http://alembic.zzzcomputing.com/en/latest/index.html) which is the migration tool dedicated to `SQLAlchemy`.
+
+## Quick summup ##
+
+In order to use the `gearbox migrate [...]` commands, change your current directory to be `tracim/` from the root of the project :
+
+    (tg2env) user@host:~/tracim$ cd tracim/
+    (tg2env) user@host:~/tracim/tracim$
+
+### Retrieving schema current version ###
+
+    gearbox migrate db_version
+
+### Creating new schema migration ###
+
+This creates a new python migration file in `tracim/migration/versions/` ending by `migration_label.py`:
+
+    gearbox migrate create 'migration label'
+
+### Upgrading schema ###
+
+    gearbox migrate upgrade
+
+### Downgrading schema ###
+
+    gearbox migrate downgrade