update readme

README.md

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 <img src="https://raw.githubusercontent.com/cdimascio/java-dotenv/master/assets/java-dotenv.png" alt="dotenv" align="right" />
 
-Dotenv is a zero-dependency module that loads environment variables from a `.env`. Storing configuration in the environment separate from code is based on The Twelve-Factor App methodology.
+Dotenv is a zero-dependency module that loads environment variables from a `.env`. Storing configuration in the environment separate from code is based on The [The Twelve-Factor App](http://12factor.net/config) methodology.
 
 **Note:** Java does not provide a way to set environment variables on a currently running process. Thus, once `java-dotenv` is configured, you can use the `dotenv.get("...")` API to get environment variables, instead of `System.getenv(...)`. `dotenv`  should be used to retrieve all environment variables. Those listed in `.env` will override those in the the environment.  
 ## Install