Minor minor issue: markdown files may not render correctly out of github

Markdown files distributed with this project (e.g. INSTALL.md) may not render correctly when displayed out of github (or better, with a markdown viewer that is not the github one).

Specifically, when paths including shell variables are used (e.g. $MANDIR/man1/) the string is not guarded by back quotes. As such all viewers supporting MathJax will interpret the $ as the beginning of a mathematical expression and misrender all the following text.

To avoid this problem, I suggest guarding all paths and shell variables in markdown files withing backquotes.