l3build: A few notes on localdir

Most of the time, it makes sense to keep 'local' files separate from
'current business', but that's more complex for the tests so everything is
done in one place. That may not be immediately apparent, so add some notes.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5136 de43f980-851b-0410-b2f7-c40aca1f87e0