adds warning

[jasonfb]

Note that even the README on 0.1.16 is confusing now, because the subdomain given to you does not work without adding /admin/api.php to the end of it

here, I've configured it globally using an all-caps global variable as the object is supposed to exist as a singleton.

ACTIVE_CAMPAIGN_CLIENT = ::ActiveCampaign::Client.new(
        api_endpoint: 'https://[YOUR ACTIVE CAMPAIGN DOMAIN]/admin/api.php', # e.g. 'https://yourendpoint.api-us1.com/admin/api.php'
        api_key: '[YOUR-API-KEY]') # e.g. 'a4e60a1ba200595d5cc37ede5732545184165e'

the gem works fine but the documentation is head-spinningly confusing, so much so that the barrier to entry for using this gem is unnecessarily high.