chore: improve test guide

CONTRIBUTING.md

@@ -32,6 +32,14 @@ To run tests:
 - :test_tube: Run `yarn check-types` to check TypeScript types.
 - :test_tube: Run `yarn lint` to run the linter (ESLint).
 
+Note that to run `test:e2e`, first ensure that the environment is correctly setup by running the `run_e2e_env.sh` script.
+
+```sh
+GETH_DOCKER_IMAGE=ethereum/client-go:v1.11.6  NETHERMIND_DOCKER_IMAGE=nethermind/nethermind:1.18.0 ./scripts/run_e2e_env.sh start
+```
+
+Similarly, run `yarn download-spec-tests` in both folders `packages/beacon-node` and `packages/validator` before running `yarn test:spec`.
+
 Contributing to tests:
 
 - Test must not depend on external live resources, such that running tests for a commit must be deterministic: