Document how to build targets in directories that start with `@`
Document how to build targets in directories that start with `@`

closes #3716

Signed-off-by: Antonio Nuno Monteiro <cd21d25d582439807a927faa56fe2def0afef02a@gmail.com>

janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder