Update to branch name to"main" for inclusive design language

[coryfink]

It is important to build our technology with inclusive design language in order to foster an inclusive workplace where everyone feels they can belong. In order to achieve this, can we change our branch name from master to main?

Changes:

1. change our branch name from master to main
2. update documentation here: https://github.com/react-hook-form/documentation/blob/master/CONTRIBUTING.md

[coryfink]

I can update documentation once an admin changes branch name!