Add a note on compatibility in README

[angelhof]

I added this note so that new users know what to expect with respect to PaSh's compatibility. @mgree, @nvasilakis feel free to edit or approve this and then we can merge it.

Signed-off-by: Konstantinos Kallas [email protected]

[mgree]

Might be good to give an indication of the kinds of things that don't work, e.g., variables in redirects, arrays, process substitutions, some fancy bash parameter forms.