doc: add primordials guidelines

PR-URL: nodejs/node#38635
Reviewed-By: Akhil Marsonya <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Robert Nagy <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Geoffrey Booth <[email protected]>