Changing central branch's name from "master" to "main"

## Problem
I was scrolling through the repo and noticed that the central branch is called "master". This can create a non-inclusive environment for Black contributors from the U.S. as the word has a connotation of slavery.

## Proposed Solution
I highly recommend changing the name to "main".

## Benefits
1. It'll create a more inclusive contributing environment
2. GitHub has made this an [official change](https://github.blog/changelog/2020-10-01-the-default-branch-for-newly-created-repositories-is-now-main/)