Improve Fish installation instructions

[ilyagr]

Most notably, this adds instructions of changing $_Z_CD. I think many
fish users would find them useful. See #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 deleted sentence is important, we could add it back, but
it would apply to all the shells, wouldn't it? We could also de-emphasize it
by making it just a warning, something like

You can instead manually insert the output of lua /path/to/z.lua --init fish
into the file, but then make sure to regenerate it every time you update or move z.lua.

Finally, some wording is improved.

[skywind3000]

thanks

[ilyagr]

Thank you!

[Flurrywinde]

I also use Fish (v.3.5.1), so I tried using the instructions in the readme to install it:

Create ~/.config/fish/conf.d/z.fish with following code

lua /path/to/z.lua --init fish | source

Shouldn't the instructions say that this line is to be added to one's ~/.config/fish/config.fish file?

Also, in the repo, there's an init.fish. What's that for? I see it makes the aliases, but it seems this file isn't ever executed?

[ilyagr]

Shouldn't the instructions say that this line is to be added to one's ~/.config/fish/config.fish file?

You're welcome to put the line in config.fish, @Flurrywinde , it will work fine. I just wanted to have one set of instructions that will work for anyone. The instructions can of course be adapted by the user according to preference. If I remember correctly, config.fish was a relatively recent addition to fish when I created this PR.

Also, in the repo, there's an init.fish. What's that for? I see it makes the aliases, but it seems this file isn't ever executed?

I'm not sure. I looked at the git log, and it seems it may have something to do with oh-my-fish or fisher. @skywind3000 , perhaps you could add a comment to init.fish explaining what it's for, or delete it?