Add documentaton for "key" property

[glensc]

what

Document using "key":

• https://github.com/cloudposse/terraform-cloudflare-zone/blob/c464ebf797e1dd8283c8c5f85e703ea82377338b/main.tf#L11

why

Default terraform resource name is based on value. I wouldn't want that for CNAMEs. The workaround is to specify fixed key for those records.

references