Helpful instructions for database updates

Affects: (install directory)/docs/INSTALL.html

Please add instructions to use the phpBB command line interpreter for database updates. The time saved using this approach vs. the web browser is very significant. This feature is not mentioned anywhere except the online guide Upgrading from 3.0 to 3.1 and should be in the released documentation.

In section "4. Updating from stable releases of phpBB 3.1.x" add after:

Please make sure you update your phpBB source files too, even if you run the database_update.php file.

If you have a large board, you may wish to update via the command line. From your board's root, execute the following command:

php ./bin/phpbbcli.php db:migrate --safe-mode