Add discussion on latexindent.pl

[tdegeus]

A draft :

Hi ATcmhughes . To be honest I have been having some frustration with latexindent.pl. In particular, I found that it was too aggressive from time-to-time, and I found myself doing a lot of handwork after. I have been reflecting on a number of style-rules that would be better suited I believe, as summarised here:

https://texplain.readthedocs.io/en/latest/module.html#texplain.indent

To convince myself I also implemented an indentation tool and have been successfully testing it on my documents.

TODO: needs some examples