Cleanup CONTRIBUTING.md file (#3550)
Cleanup CONTRIBUTING.md file (#3550)

As part of the effort to standardize working between all the qiskit
elements and adopting the same workflow and common practices for each
element we recently added most of terra's contributing.md file into the
documentation (see Qiskit/qiskit#868). This commit updates the
contributing file to reference the hosted documentation and removes the
duplicated sections. At the same time this commit also adds some additional
details on referencing issues inside a release note.
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py