Add installation section to readme

Make sure folks know this is a package in the general registry

Author: logankilpatrick

README.md

@@ -5,3 +5,12 @@
 [![Build Status](https://travis-ci.org/FluxML/MacroTools.jl.svg?branch=master)](https://travis-ci.org/FluxML/MacroTools.jl)
 
 MacroTools provides a library of tools for working with Julia code and expressions. This includes a powerful template-matching system and code-walking tools that let you do deep transformations of code in a few lines. See the [docs](http://fluxml.github.io/MacroTools.jl/) for more info.
+
+Suggestions, issues and pull requsts are welcome.
+
+## Installation
+
+```julia
+]add MacroTools
+```
+