Modify CONTRIBUTING.md #14

Short summary

Current CONTRIBUTING.md is outdated.

Long description

• Modify CONTRIBUTING.md so that it reflects the current refactor of the code.
• Add a section specifying how pull requests should be performed.
• Only this file should be modified, no others.
• It should also contain instructions on how to run the tests. Some explanation into these commands will need to be given export ANSYSEM_ROOT241=/data/AnsysEM/v241/Linux64, python -m pytest -m "" -v and python -m pytest.

Estimated time investment

3 hours