Add release-tracking AUR package to README

[deraffe]

libpurple-lurch is more popular than libpurple-lurch-git and will only track tagged releases, which is what most people probably want.

To avoid long lines the URLs have been hidden behind Markdown links.

[gkdr]

@deraffe are you interested in fixing the link? would the correct one just be https://aur.archlinux.org/packages/libpurple-lurch-git ?

generally, that repology overview is pretty great. i think the most useful thing to do would be to ask repology to merge all the different project names into one (see https://github.com/repology/repology-rules) and then use an overview like https://repology.org/badge/vertical-allrepos/purple-lurch.svg?header=lurch or textual https://repology.org/project/purple-lurch/versions. out of scope for this pr, i'll seperate this to an issue later :slightly_smiling_face: