SRI: link to integrity metadata in the Fetch spec

While we have our own "integrity metadata" section that expands on
the format of the metadata, we should also reference the Fetch spec
on that topic.

#428 (comment)

Author: Francois Marier

specs/subresourceintegrity/spec.markdown

@@ -191,7 +191,8 @@ used to verify the response.
 ### Integrity metadata
 
 To verify the integrity of a response, a user agent requires <dfn>integrity
-metadata</dfn>, which consists of the following pieces of information:
+metadata</dfn> as part of the [request][]. This metadata consists of the following
+pieces of information:
 
 * cryptographic hash function ("alg")
 * [digest][] ("val")
@@ -223,6 +224,7 @@ result of the following command line:
 
     echo -n "alert('Hello, world.');" | openssl dgst -sha256 -binary | openssl enc -base64 -A
 
+[request]: https://fetch.spec.whatwg.org/#concept-request-integrity-metadata
 [csp2-section42]: http://www.w3.org/TR/CSP2/#source-list-syntax
 [openssl]: https://www.openssl.org/
 </div>