docs: Add file limit command for macOS (nvim-lua#430)

The README only showed the command to raise the limit on the number of
open files. I added the command for macOS, as it can be difficult to
find.

README.md

@@ -391,8 +391,9 @@ DELETED: Please use packer.nvim or other lua-rocks wrapper instead. This no long
 ### FAQ
 
 1. Error: Too many open files
+
 - \*nix systems have a setting to configure the maximum amount of open file
-  handles. It can occur that the default value is pretty low and that you end
-  up getting this error after opening a couple of files. You can see the
-  current limit with `ulimit -n` and set it with `ulimit -n 4096`. (macos might
-  work different)
+  handles. It can occur that the default value is pretty low and that you end up
+  getting this error after opening a couple of files. On Linux you can see the
+  current limit with `ulimit -n` and set it with `ulimit -n 4096`. If you're on
+  macOS the command is `sudo launchctl limit maxfiles 4096 4096`.