Include instructions for mu and other modules

[pyluyten]

thanks for this flatpak! this is so great

i could not understand how to make mu work. If i try to load my local installation of mu, it seems the file is not found, which i bet is by design = sandbox, so i tempted to find mu4e in flatpak. Then i miserably failed - i can find runtimes, applications, but not modules. Do i need to look better or install something else? i believed flatpak install would also install xapian and everything but the only xapian on my system is the package manager one.

this is why i believe some basic README would help

[pyluyten]

btw i tried in init.el to perform a cond on /var/run/usr/share to check if mu4e is there, so if emacs is flatpaked i load the mu that is system-wide but mounted on /var/run , which also implies to setq mu4e-mu-binary , but then i have another error and i bet this is expected and this is why you provide the module.