Merge pull request #3006 from mitre/magma-config-readme-changes

Added paragraph to readme noting config impacts on Magma build

README.md

@@ -88,9 +88,11 @@ Finally, start the server.
 ```Bash
 python3 server.py --insecure --build
 ```
-The --build flag automatically installs any VueJS UI dependencies, bundles the UI into a dist directory, and is served by the Caldera server. You will only have to use the --build flag again if you add any plugins or make any changes to the UI.
+The --build flag automatically installs any VueJS UI dependencies, bundles the UI into a dist directory, writes the Magma plugin's `.env` file, and is served by the Caldera server. You will only have to use the --build flag again if you add any plugins or make any changes to the UI.
 Once started, log into http://localhost:8888 using the default credentials red/admin. Then go into Plugins -> Training and complete the capture-the-flag style training course to learn how to use Caldera.
 
+In some situations the default configuration values can cause the UI to appear unresponsive due to misrouted requests. Modify the `app.frontend.api_base_url` config value and start the server using the --build flag to update the UI's request URL environment variable.
+
 If you prefer to not use the new VueJS UI, revert to Caldera v4.2.0. Correspondingly, do not use the `--build` flag for earlier versions as not required.
 
 ### User Interface Development