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

# 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.
```go
type Client struct {
       ...

	CRM       *CRM
	Marketing *Marketing // <- this
}
```

# Acceptance criteria
Make how to add a domain clear.