Include documentation in the application

doc/sqlite-database.markdown

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+Sqlite database management
+==========================
+
+Kanboard uses Sqlite by default to store its data.
+All tasks, projects and users are stored inside this database.
+
+Technically, the database is just a single file located inside the directory `data` and named `db.sqlite`.
+
+Export/Backup
+-------------
+
+### Command line
+
+Doing a backup is very easy, just copy the file `data/db.sqlite` somewhere else when nobody use the software.
+
+### User interface
+
+You can also download at any time the database directly from the **settings** menu.
+
+The downloaded database is compressed with Gzip, the filename becomes `db.sqlite.gz`.
+
+Import/Restoration
+------------------
+
+There is actually no way to restore the database from the user interface.
+The restoration must be done manually when no body use the software.
+
+- To restore an old backup, just replace and overwrite the actual file `data/db.sqlite`.
+- To uncompress a gzipped database, execute this command from a terminal `gunzip db.sqlite.gz`.
+
+Optimization
+------------
+
+Occasionally, it's possible to optimize the database file by running the command `VACUUM`.
+This command rebuild the entire database and can be used for several reasons:
+
+- Reduce the file size, deleting data produce empty space but doesn't change the file size.
+- The database is fragmented due to frequent inserts or updates.
+
+### From the command line
+
+```
+sqlite3 data/db.sqlite 'VACUUM'
+```
+
+### From the user interface
+
+Go to the menu **settings** and click on the link **Optimize the database**.
+
+For more information, read the [Sqlite documentation](https://sqlite.org/lang_vacuum.html).