Expand README with warning about additional properties & contexts.

README.md

@@ -101,6 +101,16 @@ Please note: implementations may specify authorization parameters for oauth2 or
 zcaps, but not both. Implementations may also not specify any authorization
 parameters, in which case they do not specify `oauth2` or `zcap` properties.
 
+Please check specific test suite READMEs for further details on implementation properties and endpoints. 
+Test suites MAY specify additional properties and features for endpoints such as
+mandatory JSON-LD contexts, keyTypes settings, additional tags for specific tests such as Enveloped Proofs,
+and supported VC Data Model versions. Endpoints in most cases should be VC-API capable and able
+to resolve the following contexts:
+
+- https://www.w3.org/ns/credentials/examples/v2
+- https://www.w3.org/2018/credentials/v1
+- https://www.w3.org/ns/credentials/v2
+
 #### Testing locally
 
 To test implementations with endpoints running locally, create a configuration file named