README.md update - References at Contributing section

[behramulukir]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

This PR adds links to Contributing section at README.md to correctly reference the CONTRIBUTING.md and issues tab on GitHub.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Aalto University

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:
Aalto University

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[jgabry]

Great, thank you!