update xkbswitch-macosx README reference

[xiehuc]

original xkbswitch-macosx's code is full of bug and hard to understand.

for eample:

1. it use input source list index as an id, makes id differs on
   different machine
2. it show layout by hard code it. just couldn't know why
3. it couldn't set im.rime.inputmethod.Squirrel.Rime because it believes
   every sourceid has an com.apple prefix and hard code it.

so i modify it (copy from https://github.com/minoki/InputSourceSelector,
change api, which earlier and better)

[rlue]

Interesting! I can appreciate all the effort you put in to modify InputSourceSelector's API, but my preference would be to avoid forks wherever possible.

What would you think if we modified vim-barbaric to add support for InputSourceSelector as well as xkbswitch, and use whichever one it finds on the user's $PATH? We could add code to vim-barbaric to support InputSourceSelector's API directly instead of adding a dependency to your fork, which could incur some maintenance burden in the long run.

[xiehuc]

you are right, i would do it when i have time.

i would also try pull request to original xkbswitch-macos to avoid fork