Update documentation on how to write serial tests
## Writing e2e tests
We aim to run e2e tests in parallel by default. As such the following rules should be followed:
- * Use cirros and alpine VMs for testing wherever possible. If you have to use
+ * Use cirros and alpine VMs for testing wherever possible. If you have to
use another OS, discuss on the PR why it is needed.
* Stay within the boundary of the Testnamespaces which we prove. If you have
- to create resources outside of the test namespaces, discuss potential
+ to create resources outside the test namespaces, discuss potential
solutions on such a PR.
* If you really have to run tests serial (destructive tests, infra-tests,
- ...), mark the test with a `[Serial]` tag.
+ ...), mark the test with a `[Serial]` tag and add the [Serial decorator](https://onsi.github.io/ginkgo/#serial-specs) to the test.
* If tests are not using the default cleanup code, additional custom
preparations may be necessary.
