Manual update to include "damp APM with foam" recommendation

In the forum, Jason is recommending that the APM should be damped with foam in 
its mounting to reduce the noise seen by the accelerometers and gyros. Here's 
the relevant article:
http://www.diydrones.com/forum/topics/stick-centerdrift-problem?commentId=705844
%3AComment%3A418438&xg_source=msg_com_forum

The manual needs to updated to reflect this, as current instructions bolt it 
down firmly.

At the same time, could also add an instruction to cover the barometer sensor 
with felt or foam, as that seems to be recommended practice now too. The foam 
disk from an earbud headphone seems like a suitable material.

Original issue reported on code.google.com by goo...@harding99.freeserve.co.uk on 7 Jun 2011 at 2:59

coby7016 / arducopter

Manual update to include "damp APM with foam" recommendation #190