Move English documentation to folder en_US

doc/en_US/update.markdown

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+Upgrade Kanboard to a new version
+=================================
+
+Most of the time, upgrading Kanboard to a newer version is seamless.
+The process could be summarized to simply copy your data folder to the new Kanboard folder.
+Kanboard will run database migrations automatically for you.
+
+Important things to do before updating
+--------------------------------------
+
+- **Always make a backup of your data before upgrading**
+- **Check that your backup is valid!**
+- Check again
+- Always read the [change history](https://github.com/kanboard/kanboard/blob/master/ChangeLog) to check for breaking changes
+- Stop the worker if you use it
+- Put the web server in maintenance mode to avoid people use the software while upgrading
+
+From the archive (stable version)
+---------------------------------
+
+1. Decompress the new archive
+2. Copy the `data` folder into the newly uncompressed directory
+3. Copy your custom `config.php` if you have one
+4. If you have installed some plugins, use the latest version
+5. Make sure the directory `data` is writeable by your web server user
+6. Test
+7. Remove your old Kanboard directory
+
+From the repository (development version)
+-----------------------------------------
+
+1. `git pull`
+2. `composer install --no-dev`
+3. Login and check if everything is ok
+
+Note: This method will install the **current development version**, use at your own risk.
+
+Running SQL migrations manually
+-------------------------------
+
+By default, SQL migrations are executed automatically. The schema version is checked at each request.
+In this way, when you upgrade Kanboard to another version, the database schema is updated for you.
+
+You may want to disable this behaviour is case you have a specific configuration.
+For example, if multiple processes try to apply the migrations in the same time you might have some concurrency problems even if each operation is executed inside a transaction.
+
+To disable this feature, set the parameter `DB_RUN_MIGRATIONS` at `false` in your [config file](config.markdown).
+
+When you will have to upgrade Kanboard, run this command:
+
+```bash
+./cli db:migrate
+```