Adding user contribution link to README.md

This is a nice quick tutorial from @stevekm for using Docker2Singularity on MacOS with Vagrant - definitely a combination that will be useful for others!

Author: vsoch

README.md

@@ -24,6 +24,12 @@ Replace `D:\host\path\where\to\output\singularity\image` with a path on the host
 
 `docker2singularity` uses the Docker daemon located on the host system. It will access the Docker image cache from the host system avoiding having to redownload images that are already present locally.
 
+### Contributed Examples
+The following are a list of brief examples and tutorials generated by the Singularity community for using **docker2singularity**. If you have an example of your own, please [let us know](https://www.github.com/singularityware/docker2singularity/issues)!
+
+ - [docker2singularity-demo](https://github.com/stevekm/docker2singularity-demo): an example of using docker2singularity on MacOS using Vagrant, complete with notes and a nice Makefile
+
+
 ## Tips for making Docker images compatible with Singularity
 
  - Define all environmental variables using the `ENV` instruction set. Do not rely on `.bashrc`, `.profile`, etc.
@@ -61,6 +67,7 @@ If you are getting `WARNING: Non existant bind point (directory) in container: '
      singularityware/docker2singularity \            
      -m "/shared_fs /custom_mountpoint2" \
      ubuntu:14.04
-
+     
+     
 ## Acknowledgements
 This work is heavily based on the `docker2singularity` work done by [vsoch](https://github.com/vsoch) and [gmkurtzer](https://github.com/gmkurtzer). Hopefully most of the conversion code will be merged into Singularity in the future making this container even leaner!