Include documentation in the application

doc/update.markdown

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+Update
+======
+
+**Always make a backup of your database before upgrading!**
+
+From the archive (stable version)
+---------------------------------
+
+1. Close your session (logout)
+2. Rename your actual Kanboard directory (to keep a backup)
+3. Uncompress the new archive and copy your `data` directory to the newly uncompressed directory.
+4. Copy your custom `config.php` (if you created one) to the root of the newly uncompressed directory.
+5. Make the directory `data` writeable by the web server user
+6. Login and check if everything is ok
+7. Remove the old Kanboard directory
+
+
+From the repository (development version)
+-----------------------------------------
+
+1. Close your session (logout)
+2. `git pull`
+3. `composer install`
+3. Login and check if everything is ok
+
+Note: This method will install the **current development version**, use at your own risk.