Update the contributing to QuantEcon.jl page?

[natashawatkins]

Maybe some people here could improve the pages here: https://quantecon.org/julia-developers

[arnavs]

Initial thoughts:

• Point people to the lecture-source-jl repo (which we should probably clean up for new contributors).

• Remove stuff about style guides not existing. There's a good one on the Julia website and on the lectures repo. The linked PR for docstrings has also been merged in.

• Remove the stuff about test guides not existing, and instead replace it with a short intro to unit and regression tests. (edit: The testing lecture in the JL notes would be good, but has a lot of extraneous stuff about Travis/Codecov/Package manager, so this should probably a small Markdown file or something).

• Link people to the first few lectures in the Julia side (i.e., getting started, version control, tools_editors) so that people are working with a consistent setup.