Document changes in this fork

[jirutka]

Please document changes made in this fork, i.e. how it differ from https://github.com/LuaJIT/LuaJIT.

[agentzh]

@jirutka That's a good idea. Already listed the changes in README:

https://github.com/openresty/luajit2#readme

Thanks for the suggestion.