README: Recommend installing Marathon directly from its repo

Homebrew version is not currently supported.

Author: JohnSundell

README.md

@@ -10,7 +10,9 @@ The easiest way to install this Xcode theme is using **[Marathon](https://github
 
 1. Install the latest version of Marathon if you haven't already:
 ```
-$ brew install marathon-swift
+$ git clone https://github.com/JohnSundell/Marathon.git
+$ cd Marathon
+$ make
 ```
 
 2. Run the install script which will install [Adobe's Source Code Pro](https://github.com/adobe-fonts/source-code-pro) font & this Xcode theme for you: