Give broad definition of "renewal" at top of doc (#62)

Fixes #58

draft-ietf-acme-ari.md

@@ -45,6 +45,8 @@ This document specifies a mechanism by which ACME servers may provide suggested
 
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [@!RFC2119;@!RFC8174] when, and only when, they appear in all capitals, as shown here.
 
+Throughout this document, the word "renewal" and its variants are taken to encompass any combination of "Renewal", "Re-Key", and "Modification" as defined in [@!RFC3647].
+
 # Extensions to the Directory Object
 
 An ACME server which wishes to provide renewal information **MUST** include a new field, `renewalInfo`, in its directory object.