Add more details to the E2E Test README. Configure dev/test to use MPAC (#1869)

instances of the new backend services.

src/ConfigContext.ts

@@ -199,6 +199,9 @@ if (process.env.NODE_ENV === "development") {
   updateConfigContext({
     PROXY_PATH: "/proxy",
     EMULATOR_ENDPOINT: "https://localhost:8081",
+    PORTAL_BACKEND_ENDPOINT: PortalBackendEndpoints.Mpac,
+    MONGO_PROXY_ENDPOINT: MongoProxyEndpoints.Mpac,
+    CASSANDRA_PROXY_ENDPOINT: CassandraProxyEndpoints.Mpac,
   });
 }