fbjs assumes yarn is installed but doesn't declare it as dependency

[claudiopro]

fbjs assumes in various places yarn is installed but doesn't declare it as a dependency or call it out in the README. To name a few:

• Build scripts -

  fbjs/package.json

  Lines 18 to 22 in af9fefa

  	"fbjs:build": "cd packages/fbjs && yarn && yarn run build",
  	"fbjs:test": "cd packages/fbjs && yarn && yarn run test",
  	"fbjs:flow": "cd packages/fbjs && yarn && yarn run flow",
  	"preset:test": "cd packages/babel-preset-fbjs && yarn && yarn run test",
  	"signedsource:test": "cd packages/signedsource && yarn && yarn run test"

• fbjs prepublish script -

  fbjs/packages/fbjs/package.json

  Line 11 in af9fefa

  "prepublish": "yarn run build",

• check-dependencies Gulp plugin -

  fbjs/packages/fbjs-scripts/gulp/check-dependencies.js

  Line 25 in ccad8f1

  'yarn',

etc.

I believe it would be useful to call out fbjs depends on Facebook's omakase stack in the README at a minimum.

[zpao]

Ah, I totally missed that. Looks like it snuck in in #303. We should probably just fix that so it uses npm again. Or fallback to yarn. Or better, work out if you're using yarn in the caller. There's no reason for it to actually require yarn though.

That said, draft is using yarn for it's dependency manager so if you're working on draft, it should use yarn.

[zpao]

I don't particularly care to call it out for contributing in this project since we're fairly unfriendly to external contributions anyway (really not intended to be good at it). But we shouldn't break people trying to contribute to our projects.