manual revisions

[porres]

• [x] fix [float] description as it says it only takes bang and float (not true anymore)
• [x] improve section on 'selectors'
• [x] document about numeric/float precision and rounding
• [x] rename "Anatomy of a message" to "selectors and float precision"
• [x] better document how to install pd64 and how you can run both single and double on the same machine
• [x] Improve "compiled externals" section and remove info on PowerPC (I think it is safe to let it go by now)
• [x] add pd~ for MAX picture