Draft Contributing file for NVML. 

 I'm uploading it here because I have a few questions. 

1) I included the CHECKLIST content, tweaked for web presentation.  Do we want to maintain 2 copies?  Or just have Contributing.md refer to the CONTRIBUTING text file?

2) Do we want to say anything about style expectations?  Or just hope contributors run make cstyle?

3)  Closing bugs in commit messages is a problem for validation.  I suggest we tell developers to not use the commit message / issue close integration features (and remove related text from the Contributing file)