[contributing] Adds Step To Unit Testing About Mocking (#8240)
[contributing] Adds Step To Unit Testing About Mocking (#8240)

# Why

I was working on a [PR](https://github.com/expo/expo/pull/8199/) for a package that did not have unit tests yet. It took me a few min to figure out where to add the mocks for the native functions.
To help others, this adds a note to the contributing instructions.

# How

Updated the contributing document. 

# Test Plan

None.

exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent