README: Include note about merged Subtargets

When adding support for a previously-generic device, the
correct MIPS MACH has to be added. The compiled image
will lead to bootloops otherwise.

Because this might not be obvious, a note is added to the
READMEs section about supporting a new device.

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>