Add instructions for archlinux

[2m]

Archlinux now has a jabba package which installs only the jabba binary.

That needs to be coupled with a plugin for a corresponding shell for jabba to work. For example, a plugin for oh-my-zsh: ohmyzsh/ohmyzsh#6886

[shyiko]

Just so that it would be clear: I'm not ignoring this PR. I'll merge it in as soon as ohmyzsh/ohmyzsh#6886 in closed.

[2m]

Of course. But oh-my-zsh repo has >800 open PRs. I should probably put jabba plugin somewhere else.

[2m]

It seems that PRs are not being merged in the oh-my-zsh repository. I have packaged up zsh jabba plugin to a separate repository (https://github.com/2m/zsh-jabba) and fixed up README in this PR.

[2m]

I also packaged up the fish integration script and a bit of completion support here: https://github.com/2m/fish-jabba

Also updated this PR to mention fish plugin.

[ghost]

@shyiko Can we get this merged as soon as Marty addresses my comments in the PR?

[2m]

Addressed the feedback and rebased on top of the latest master.