[INTERNAL] Update GitHub contributing documentation
[INTERNAL] Update GitHub contributing documentation

The previously used link was not visible to the public

Closes https://github.com/SAP/openui5/pull/469
Change-Id: I1a04b27d6e6badf5a2734dc7ef5fc3dda878162b

olirogers/openui5,nzamani/openui5,cschuff/openui5,SAP/openui5,SQCLabs/openui5,SAP/openui5,SQCLabs/openui5,cschuff/openui5,olirogers/openui5,SAP/openui5,nzamani/openui5,olirogers/openui5,SQCLabs/openui5,SAP/openui5,nzamani/openui5,cschuff/openui5