Add git alias usage to docs

Why is this change necessary?

* A git alias has the benefit of being less keystrokes than having to
  write the piped series of commands each time, and leaves the default
  behaviour of `git diff` intact.
* git aliases are tab-completed by git's default bash autocompletion.

How does it address the issue?

* Adds a section to the 'Usage' section of the README, with a git alias
  definition.

readme.md

@@ -24,6 +24,11 @@ git diff --color | diff-so-fancy
 git config --global core.pager "diff-so-fancy | less --tabs=1,5 -R"
 ```
 
+Or, create a git alias  in your `~/.gitconfig` for shorthand fanciness:
+```shell
+diff-so-fancy = "!git diff --color $@ | diff-so-fancy"
+```
+
 ## Install
 
 For convenience, the recommended installation is via NPM: