Installation.md: Zsh-plugin method which doesn't require root access

[psprint]

A short section to Installation.md. It describes how to install as Zsh plugin. Benefits of such method:

• no administrator access required (easy install on e.g. digital ocean, etc.),
• declarative method of installing – 2 lines in .zshrc lead to git-extras on each account.

[spacewander]

@psprint
So the zplugin will take over the upgrade of git-extras?
Should we also apply a patch as what we have done with Homebrew?
https://github.com/tj/git-extras/blob/master/brew-release.patch
https://github.com/tj/git-extras/blob/master/.travis.yml#L9

[spacewander]

Now I think we should have the same patch instead of xxx-release.patch. We could use some unix command to update the patch according to the package manager specified via Makefile arguments.

[psprint]

Yes Zplugin will update. Here is more detailed description. I've tested git-extras update and it works normally, because files are in $HOME, writable. Issuing zplugin update tj/git-extras will update too, not to latest tag but to HEAD, but that's not a problem. So I think the patch isn't needed.

[spacewander]

@psprint
Good news to know. Already merged, thanks!