Documentation improvement suggestions

[graeme-winter]

This script was a big help to me, thank you

I would note a couple of documentation improvements:

• it would be very helpful to be explicit that EPICS_BASE should be available before this is started
• it would appear there are build dependencies: sudo apt install re2c libxcomposite-dev libusb-dev got me over the line

Also, simply running the script with

./assemble_synApps/assemble_synApps --base $EPICS_BASE

at first gave a weird error:

EPICS/Release.pm: Undefined variable $(EPICS_BASE) used 
EPICS/Release.pm: Undefined variable $(EPICS_BASE) used 
EPICS/Release.pm: Undefined variable $(EPICS_BASE) used 

Going into git/support and typing make eventually cleared by installing all the dependencies. This got me to a working base.

[graeme-winter]

Some overlap on dependencies with #6 noted