Work on braille translation for Vietnamese

[NSoiffer]

Vietnamese braille is similar to UEB, but not the same. There is a small start in Rules/Braille/Vietnam, but it mainly a copy of the UEB directory.

Vietnamese Braille Rules for Maths and Chemistry.docx is a document with important info.

I had some questions and @danghoaiphuc responded with these answers.

[danghoaiphuc]

Thanks @NSoiffer for raising this issue. I just PR with some definitions and adding rules based on your existing ones for \tan, \cotan, arcsin etc. If you could help to implement rules for number 2 mentioned in the answer file above, I think it's quite good to be ready for Vietnamese users to use. Number 2 is all about Vietnamese capital rules, Roman and Greek letters. Again, many thanks for your support.