Add non-numeric name constraint

ifa-0001.md

@@ -53,6 +53,8 @@ The key of the key-value pair SHALL be an ASCII-encoded name in the form `d/NAME
 
 Note that as per the above regex, the standard rules for domain names are enforced. Domain names MUST NOT begin or end with a hyphen, and consecutive hyphens MUST NOT appear except as permitted by the Internationalized Domain Names (IDN) specification (RFC 3492).
 
+As an additional constraint, wholly numeric names, such as `d/123`, are not permitted. Names which begin with a digit but which are not wholly numeric, such as `d/123four` are permitted. (Historically, in the ICANN DNS system, fully numeric names such as `2600.org` were permitted. However such registrations are no longer allowed, although existing registrations such as `2600.org` itself are grandfathered. Namecoin adopts this constraint firstly for parity with contemporary domain naming policy, and secondly to enable the future assignment of all-numeric domains to denote name epoch for secure disambiguation purposes.)
+
 The `d/` prefix identifies the Domain Names namespace within the Namecoin key namespace. Names under this prefix are reserved for use in relation to this specification and preceding versions of it. Keys not beginning with that prefix are wholly unrelated to this specification and are not required to conform to it.
 
 The part of the name following the `d/` prefix is the name which manifests in the .bit TLD. A key of `d/example` registers the name `example.bit.`