See https://github.com/riscv/riscv-opcodes/issues/204



Before this change the text implied that bit encodings and variable arguments could not be mixed in the list of instruction arguments. 

Signed-off-by: Thomas Hepworth <tomhepworth@hotmail.co.uk>

- the python file is well commented
- the README provides a brief overview of how the python script works and the various artifacts it can generate

Closes #6