chore: clarify IDs and explain how to create a company

[Startouf]

it can be confusing which "user ID" we are talking about

• Many users of this intercom library have their own users in their own database, and their "customer ID"
• Intercom has its own unique user ID

Also, there was no example to create a company.

[Startouf]

I'm thinking of updating my PR to use "external id" to better match intercom documentation. I also found out one had to use intercom.find(external_id: user_in) and not (user_id: user_id)