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" to your ~/.config/darktable/luarc file which will enable the example script in official/hello_world.lua (note the lack of the .lua suffix).
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.