Add a document of how to add new domain in the client

[ttyfky]

What to do

Add a document of how to add new domain in the client

Background

When adding a new domain in client like below, gohubspot_test.go needs to be updated in unique way otherwise the tests will fail.

type Client struct {
       ...

    CRM       *CRM
    Marketing *Marketing // <- this
}

Acceptance criteria

Make how to add a domain clear.