docs: Add hyperlane registry as prerequisite for e2e tests (#4263)

### Description

E2E tests will fail if `hyperlane-registry` repo is not cloned next to
the monorepo. This change adds it to the `rust/README.md` file.

### Testing

Checked how `rust/README.md` looks like in GitHub.

---------

Co-authored-by: Danil Nemirovsky <4614623+ameten@users.noreply.github.com>
Co-authored-by: Trevor Porter <tkporter4@gmail.com>

rust/README.md

@@ -88,6 +88,8 @@ env $(cat ./config/validator.fuji.env | grep -v "#" | xargs) ./target/debug/vali
 
 #### Automated E2E Test
 
+Clone `hyperlane-registry` repo next to `hyperlane-monorepo` repo.
+
 To perform an automated e2e test of the agents locally, from within the `hyperlane-monorepo/rust` directory, run:
 
 ```bash