Add info about drag / drop .txt file

Most of the tools allows to drag / drop a text file that is internally converted to JCAMP-DX file format but the text format is nowhere written.

Here are the rules to parse the text file:
- only the lines containing numbers are considered
- we split the lines based on comma, semi-column or tabulation
- we take the first and second column to create the JCAMP-DX file

@remisch Please could you add a 'tip' that can open everywhere we discuss about drag / drop a txt file ?