Fixed test_bibtex unit test, updated CONTRIBUTING.md

[dlebedinsky]

Fixes #395.

Description

the test_bibtex function in the test module was attempting to read a file that did not exist. I updated it to read the real file name, reran the test, and it worked. I also updated CONTRIBUTING.md to address some pitfalls that I ran into when I tried to submit my first pull request yesterday. Hopefully this will prevent other newcomers from making the same mistakes I did!

Checklist

• [ x] Check that the base branch is set to develop and not main.
• [x ] Ensure that the documentation will be consistent with the code upon merging.
• [x ] Add a line or a few lines that check the new features added.
• [x ] Ensure that unit tests pass. If you don't have a premium proxy, some of the tests will be skipped. The tests that are run should pass without raising MaxTriesExceededException or other exceptions.

[dlebedinsky]

I just pushed another commit, in which I added a unit test for freeproxy. It is the only proxy I have, and I assume this is the case for many others, so I find it helpful. Do you see it? I assumed I would have to open a separate pull request, but I'm not sure now.