Update contribution guidelines

[Josh Soref]

When creating a PR, there's a link to the contributing file (which afaict was written on 2015?):

omi/CONTRIBUTING.md

Lines 1 to 3 in 5e68830

 Currently, this project is not accepting contributions or pull requests.  
 If you would like to get involved in this project, have a proposal for improvement or have discovered a problem,  
 please create an [issue](https://github.com/Microsoft/omi/issues "issues").  

Afaict, this repository is taking and merging PRs, so, here's an issue per the file asking that the file be updated.

Per the file, I'm not making a PR to change it.

[Jumping Yang[Wicresoft]]

@jsoref the conributing guideline is still same as mention in omi/CONTRIBUTING.md , thanks.