Adding setting up new test instruction

[huguesdevimeux]

Now that #133 will be merged soon, we have to think of how to explain to future contributors how to set up tests if a new functionality is added.

But I don't know where/how to do it. Maybe in the wiki ?

[Aathish04]

I would suggest mentioning adding tests in https://github.com/ManimCommunity/manim/blob/master/CONTRIBUTING.md
and https://github.com/ManimCommunity/manim/blob/master/README.md

And adding detailed steps in the Wiki.

[leotrs]

Does #155 close this?

[huguesdevimeux]

Yes !