Add notes on Internal vs Private visibility

[joelochlann]

Since GitHub introduced internal repositories, it's unclear whether Internal or Private is the best choice for new repositories.

It's also unclear what the role of the @guardian/guardian-developers-read team is, given the existence of internal repositories.

I've added some guidelines indicating:

1. a preference for Internal over Private
2. a preference for also granting write permissions to @guardian/guardian-developers-write
3. that @guardian/guardian-developers-read is effectively deprecated given the preference for internal repositories

[adamnfish]

Sounds great to me, thank you!