Clarify access scope for publicly visible access token

Summary:
This token is restricted to public_repo access rights and it's alright to store in the open:

```
$ curl https://api.github.com/user?access_token={...}
X-OAuth-Scopes: public_repo
```
Closes https://github.com/facebook/react-native/pull/12569

Differential Revision: D4629308

Pulled By: hramos

fbshipit-source-id: 581a4c581f7b07068eaf1e953f62fc85ceb05ab2