Add CONRIBUTING.md

[dlemstra]

Add CONRIBUTING.md to this project that explains that a personal access token is required to install @dlemstra/magick-native when running npm install.

[lancejpollard]

@dlemstra What do you mean personal access token, of what kind? Is this a GitHub or npm access token? Can't the module just be made into a GitHub repo and published without a token? Can you just explain here for now what to do? Thank you!

$ npm install
npm ERR! code E401
npm ERR! 401 Unauthorized - GET https://npm.pkg.github.com/download/@dlemstra/magick-native/0.202312.280700/45b5fea4f698c8757ccc13b8a52aa5b956ac84d8 - authentication token not provided

[dlemstra]

I have finally added this file.