Add documentation for sample applications

docs/local-development.md

@@ -56,6 +56,25 @@ If not using macOS:
 
     You should now be able to go to open up your favorite browser, go to `localhost:3000` and see your local development environment running.
 
+### Simulating a partner authentication request
+
+Typically, a person who uses Login.gov will arrive from a partner application, and their experience
+on Login.gov will be customized to incorporate the name and logo of the partner. They will also be
+asked to consent to share their information with the partner before being sent back.
+
+To simulate a true end-to-end user experience, you can either...
+
+- Use the built-in test controller for SAML logins at http://localhost:3000/test/saml/login
+- Or, run a sample partner application, which is configured by default to run with your local IdP instance:
+   - OIDC: https://github.com/18F/identity-oidc-sinatra
+      - Runs at http://localhost:9292/
+   - SAML: https://github.com/18F/identity-saml-sinatra
+      - Runs at http://localhost:4567/
+
+Running the sample application requires a few additional steps, but can be useful if you want to
+test the experience of a user being redirected to an external site, or if you want to configure
+different options of the authentication request, such as AAL or IAL.
+
 ### Running tests locally
 
   Login.gov uses the following tools for our testing: