Add a note about running frontend with test fixture data

Author: carols10cents

README.md

@@ -28,9 +28,18 @@ yarn run start:staging
 This will run a local frontend using the staging backend (hosted on Heroku at 
 [staging-crates-io.herokuapp.com](https://staging-crates-io.herokuapp.com)).
 
-If you'd like to run the frontend with a specific backend endpoint, you can 
-specify arguments to `yarn start`. For example you can set the proxy to 
-`https://crates.io/` to use the production version. 
+If you want to set up a particular situation, you can edit the fixture data used
+for tests in `app/mirage/fixtures`. Note that the fixture data does not contain
+JSON needed to support every page, so some pages might not load correctly. To
+run the frontend and use that data, don't specify any backend:
+
+```bash
+yarn run start
+```
+
+If you'd like to run the frontend with a specific backend endpoint, you can
+specify arguments to `yarn start`. For example you can set the proxy to
+`https://crates.io/` to use the production version.
 
 **Note: it is also possible to make changes to the production data**