Enable external lp8.

With this change, an external lp8 will be used if available. If no
external lp8 is available, the lp8 loader (`lua/lp8/_main.cfg`) will
fall back to its internal lp8 copy.
Update and expand readme.

Most importantly, no longer instruct all players to install via
Wesnoth’s add-ons manager, but rather provide additional instructions
for installing via Git.

Also correct some terminology (e.g., change “add-on manager” to “add-ons
manager”).