Add support for listing cascade options

Specify protocol to find out what cascade options are available to a
client using `OPTIONS` method and a specific HTTP response header. This
enables clients to programmatically determine the alternative routing
systems to which a lookup may optionally be cascaded.

Add extra wording to clarify that there are no constraints on the order
by which results are returned at the presence of `cascade` query option.

IPNI.md

@@ -453,6 +453,22 @@ to fetch the advertisement chain.
 
 An indexer node can be queried over HTTP for a multihash or a CID.
 
+
+#### Cascading Lookup
+
+The HTTP query API supports cascading queries for a given multihash or CID onto alternative routing systems in addition to searching IPNI records.
+A client may optionally specify a query parameter with key set to `cascade`, and value set to comma separated alternative routing systems, which are also searched for records.
+
+The specification imposes no constraints on the order by which the results are returned.
+Implementers are free to return results as they are found.
+
+The alternative routing systems currently supported is:
+ * `ipfs-dht`: equivalent to searching records on the IPFS network.
+
+A client may discover the list of alternative routing systems supported by a server via sending `OPTIONS` request.
+In response, the server may include `X-IPNI-Allow-Cascade` header key, with value as the comma separated list of alternative routing systems supported.
+The absence of this header key implies that the server does not offer cascading lookups.
+
 #### `GET /cid/{cid}`
 
 Given a CID as path parameter, returns a list of its content providers. The lookup ignores CID codec
@@ -465,8 +481,7 @@ and uses the multihash portion of the CID only.
 
 ##### Query Parameters
 
-* `cascade` - _Optional_. The comma separated alternative routing systems to which the lookup is cascaded in addition to searching IPNI index records. If unspecified, only IPNI index records are searched. Supported values:
-    * `ipfs-dht`- The IPFS Kademlia DHT.
+* `cascade` - _Optional_. The comma separated alternative routing systems to which the lookup is cascaded in addition to searching IPNI index records. If unspecified, only IPNI index records are searched. See [Cascading Lookup](#cascading-lookup)
 
 ##### Response
 
@@ -522,8 +537,7 @@ Given a multihash as path parameter, returns a list of its content providers.
 
 ##### Query Parameters
 
-* `cascade` - _Optional_. The comma separated alternative routing systems to which the lookup is cascaded in addition to searching IPNI index records. If unspecified, only IPNI index records are searched. Supported values:
-  * `ipfs-dht`- The IPFS Kademlia DHT.
+* `cascade` - _Optional_. The comma separated alternative routing systems to which the lookup is cascaded in addition to searching IPNI index records. If unspecified, only IPNI index records are searched. See [Cascading Lookup](#cascading-lookup)
 
 
 ##### Response
@@ -592,8 +606,7 @@ Base58 string representation.
 
 ##### Query Parameters
 
-* `cascade` - _Optional_. The comma separated alternative routing systems to which the lookup is cascaded in addition to searching IPNI index records. If unspecified, only IPNI index records are searched. Supported values:
-    * `ipfs-dht`- The IPFS Kademlia DHT.
+* `cascade` - _Optional_. The comma separated alternative routing systems to which the lookup is cascaded in addition to searching IPNI index records. If unspecified, only IPNI index records are searched. See [Cascading Lookup](#cascading-lookup)
 
 
 ##### Response