Docs: Update frontend integration tests info

spec/frontend_integration/README.md

@@ -11,6 +11,33 @@ Frontend integration specs:
 
 As a result, they deserve their own special place.
 
+## Run frontend integration tests locally
+
+The frontend integration specs are all about testing integration frontend bundles against a
+mock backend. The mock backend is built using the fixtures and GraphQL schema.
+
+We can generate the necessary fixtures and GraphQL schema by running:
+
+```shell
+bundle exec rake frontend:fixtures gitlab:graphql:schema:dump
+```
+
+Then we can use [Jest](https://jestjs.io/) to run the frontend integration tests:
+
+```shell
+yarn jest:integration <path-to-integration-test>
+```
+
+If you'd like to run the frontend integration specs **without** setting up the fixtures first, then you
+can set `GL_IGNORE_WARNINGS=1`:
+
+```shell
+GL_IGNORE_WARNINGS=1 yarn jest:integration <path-to-integration-test>
+```
+
+The `jest-integration` job executes the frontend integration tests in our
+CI/CD pipelines.
+
 ## References
 
 - https://docs.gitlab.com/ee/development/testing_guide/testing_levels.html#frontend-integration-tests