Replace link to Explainer with Developer Documentation (#674)

* Replace link to Explainer with Dev Docs

The explainer has been deleted. The link to it is replaced with the MDN documentation

* Update README.md

* Update README.md

Co-authored-by: Ted Thibodeau Jr <tthibodeau@openlinksw.com>

---------

Co-authored-by: Ted Thibodeau Jr <tthibodeau@openlinksw.com>

README.md

@@ -2,7 +2,7 @@
 
 This is the repository for the W3C's FedID CG Federated Credentials Management API.
 
-Explainer: [explainer.md](explainer.md)
+Developer documentation: [Federated Credential Management (FedCM) API](https://developer.mozilla.org/en-US/docs/Web/API/FedCM_API)
 
 Work-in-progress specification: <https://w3c-fedid.github.io/FedCM/>
 
@@ -21,8 +21,9 @@ the removal of third-party cookies on federated login. Historically this has
 relied on third-party cookies or navigational redirects in order to function
 as they were the primitives provided by the web.
 
-The [explainer](explainer.md) and [spec](https://w3c-fedid.github.io/FedCM)
-provide a potential API and the rational behind how that API was designed.
+The [documentation](https://developer.mozilla.org/en-US/docs/Web/API/FedCM_API)
+and [spec](https://w3c-fedid.github.io/FedCM) provide a potential API and the
+rationale behind that API's design.
 
 ## Contributing