Some subtle issues about keywords

[llinjupt]

Hi Stevenj,

Thanks for your great work first, which is pretty usful and helpful.

I have been tring to do some interesting work with this tiny but smart tool GenPinoutSVG.
After revewing the whole REAME.md, I think there are maybe some subtle issues list below and would like
to let you know and fix them.

1. There is no keyword LABEL, instead there is LABELS. Maybe the README.md need to be updated.

2. There is nothing about "END MESSAGE" in the example csv file.

3. Also the keyword BOXES is not introduced in the README.md.

I have been writing a short HOW-TO including lots of examples. Maybe it is more helpful just in my own opnion.
After finishing, I will let you know and would like you to review and add a link to the README.md to make it handier to dive in.

[stevenj]

I am open to PR's if you want to do some updates to fix these issues, I will happily merge them. You can also PR a link to your how-to as well, I would be happy to merge that too.