The Lua scripts in this repository are meant to be used together with darktable. Either copy them individually to ~/.config/darktable/lua (you might have to create that folder) or just copy/symlink the whole repository there. That allows to update all your scripts with a simple call to git pull.
To enable one of the scripts you have to add a line like require "official/hello_world" which would enable the example script in official/hello_world.lua.
Each script includes its own documentation and usage in its header, please refer to them.
In order to have your own scripts added here they have to be under a free license (GPL2+ will definitely work, others can be discussed). Scripts in the official/ subfolder are maintained by the darktable community, those under contrib/ are meant to have an "owner" who maintains them.