Adds one line install using Sinister

Although Sinister asks for a sudo without needing it, it is just a
matter of sending a pull request to it. All in all, it is way easier to
install using it for people who do not know their way into the shell
that much.

Short URL `https://git.io/git-pair` will always point to the master
`git-pair` file. I guess there is no renaming it or I am hoping symbolic
links will work if necessary - thinking about the file name, I hate how I
missed the .sh extension to it. Oh my, lets hope that ship has not
sailed yet.

README.md

@@ -20,7 +20,11 @@ author names to your global `user.name` configuration on Git.
 
 ## Installation
 
-You can clone the project where you want to, all you need after
+One-liner using [Sinister](https://github.com/jamesqo/sinister):
+
+    $ sh <(curl -sSL http://git.io/sinister) --local --url https://git.io/git-pair
+
+Or you can clone the project where you want to, all you need after
 is to sym-link the `git-pair` script to your `$PATH`:
 
     $ cd <my-src-dir>