fix(build): add instructions for creating a token

[tido64]

Description

Added instructions for how to create an access token.

Test plan

First, move ~/Library/Preferences/rnx-build-nodejs/config.json elsewhere.

% cd incubator/build
% yarn build --dependencies
% yarn rnx-build -p ios --device-type simulator ../../packages/test-app
Missing personal access token for GitHub.

Please create a fine-grained personal access token, and save it in '~/Library/Preferences/rnx-build-nodejs/config.json' like below:

	{"github":{"token":"github_pat_0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"}}

The token must have `action:write` permission. When creating a new fine-grained personal access token, under 'Repository permissions', make sure 'Actions' is set to 'Read and write'.

For how to create a personal access token, see: https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/creating-a-personal-access-token