Main README,md does not link to CONTRIBUTING.md

[Steveolas]

On the README.md file on the organization page https://github.com/cBioPortal. the relative link to CONTRIBUTING.md is faulty as the file does not exist in https://github.com/cBioPortal/.github/blob/main/. The file does exist though in here: https://github.com/cBioPortal/cbioportal/blob/master/CONTRIBUTING.md.

Two possible solutions are to:

1. Create a CONTRIBUTING.md file in the cBioPortal organization directory.
2. Change the in the README.md file to link to the correct place

I think the first solution is better this way the README.md files remain consistent.

[Steveolas]

As the README.md files in the https://github.com/cBioPortal organization page is different than the README.md file in the https://github.com/cBioPortal/cbioportal repository I have opted to fix the issue with the first solution I suggested. I have created a pull request to fix this.

This is my first pull request ever so any feedback on how things could have been done better will be appreciated!
Ilan