Create database.md

doc/database.md

+# Duniter Database
+
+Duniter exploits an SQLite database to store the blockchain and newcoming data.
+
+You can easily set it up and explore it with the following steps:
+
+1. [Install your node](./install-a-node.md)
+2. Launch Duniter and connect it to an existing currency (for example on `duniter.org` port `8999`)
+3. Shut down Duniter
+4. Use your favorite SQLite browser (for example [Sqliteman](http://sqliteman.yarpen.cz/)) and open the file under `~/.config/duniter/duniter_default/duniter.db`
+
+Explore!
+
+<img src="https://forum.duniter.org/uploads/default/original/2X/f/f4946056f30893f33ad0b516346b02fcc78b77d5.png" width="620" height="500">