auto-completion file for the FISH shell

[eshapard]

Checklist

• I have made a change to this repository, be it functionality, testing, documentation, spelling, or grammar.
• I updated my branch with the master branch.
• I have added the necessary testing to prove my fix is effective/my feature works (or I did not modify functionality).
• I have added necessary documentation about the functionality in an appropriate .md file.
• I have appropriately commented any code I have modified

Short description of what this PR does:

Adds an auto-completion file for the Fish shell.

[pimterry]

I'm not a fish user, but at a glance the code looks reasonable to me. Could you add some installation instructions in the readme too, with the other autocomplete docs? Here: https://github.com/pimterry/notes#installing-auto-completion

[eshapard]

Added instructions for Fish autocompletion file in README.md

[pimterry]

Great stuff, thanks!