Improve Fish installation instructions

Most notably, this adds instructions of changing $_Z_CD.  I think many
`fish` users would find them useful.
See https://github.com/skywind3000/z.lua/pull/131 for more background.

This makes the installation instructions more conventional (`|source` 
instead of `psub`). This also deletes the second (equivalent) set of instructions.
If the warning in that sentence is important, we could add it back, but
it would apply to all the shells, wouldn't it?

Finally, some wording is improved.

README.md

@@ -105,17 +105,17 @@ z -b foo    # cd to the parent directory starting with foo
 
   To generate old posix compatible script.
 
-- Fish Shell:
+- Fish Shell (version `2.4.0` or above):
 
   Create `~/.config/fish/conf.d/z.fish` with following code
 
-      source (lua /path/to/z.lua --init fish | psub)
+      lua /path/to/z.lua --init fish | source 
+  
+  If you'd like `z.lua` to cooperate with fish's own [directory history](https://fishshell.com/docs/3.2/index.html#id34), you can put
 
-  Fish version `2.4.0` or above is required. 
+      set -gx _ZL_CD cd
 
-      lua /path/to/z.lua --init fish > ~/.config/fish/conf.d/z.fish
-
-  This is another way to initialize z.lua in fish shell, but remember to regenerate z.fish if z.lua has been updated or moved.
+  into the same file.
 
 - Power Shell: