Using sources.list.d instead of endlessly appending to sources.list

There's a "new way" to using all of your custom repo lists, and that's to append it to a special fine in the  /etc/apt/sources.list.d/ directory. This helps keep things a tidy and organized.

Simply add any sources you would like to use to a separate file within this directory.

Don't be afraid if you get this message:

N: Ignoring file 'percona' in directory '/etc/apt/sources.list.d/' as it has no filename extension

It just means you need to rename the file and append ".list" to the end of it.