Add curl showing how to register a domain

curl/register_domain.sh

@@ -0,0 +1,10 @@
+# Usage: TOKEN=token ./register_domain.sh account_id domain_name attributes
+# 
+# Where `account_id` is the numeric ID of the account you are operating on, `domain_name` is
+# the name you are registering, and `attributes` is a JSON structure representing the registration
+# attributes.
+# 
+# For example:
+# 
+# `TOKEN=token ./register_domain.sh 111 example.co.uk '{"registrant_id":"111","extended_attributes":{"uk_legal_type":"FIND","registered_for":"Your Name"}}}'
+curl -i "https://api.sandbox.dnsimple.com/v2/$1/registrar/domains/$2/registrations" -H "Authorization: Bearer ${TOKEN}" -H "Accepts: application/json" -H "Content-Type: application/json" -X POST -d "$3"