Add Docker documentation

Added a section in README.md with the steps necessary to run in docker as asked in #33.

README.md

@@ -29,6 +29,18 @@ Mailhound is a simple server side script for receiving form posts and emailing t
   2. `export KEY:[email protected]`
 5. Reconnect to SSH and run the script `mailhound`
 
+### Docker Install
+
+1. Build the docker image with `docker build -t mailhound github.com/chadfawcett/mailhound`
+2. Create a config file with your environment variables
+
+    ```
+    [email protected]
+    SMTP=smtp://postmaster:[email protected]
+    ```
+
+3. Run docker with `docker run --env-file=<path to the config file> -p 8000:8000 mailhound`
+
 ## Usage
 
 ### Typical Setup