Fix waveform generation using vcs-uvm. Add waveforms section to README.

Created by: zchamski

Update user-level instructions. Re-introduce waveform generation in vcs-uvm mode.

• README.md: Add section on waveform generation. Improve readability.
• verif/sim/Makefile (vcs-testharness): Add comment and TODO. (veri-testharness): Save waveforms prior to disassembling the hex log. (questa-testharness): Add TODO comment. (vcs-uvm): Save waveforms prior to disassembling the hex log.