chore: add reminder of how to run a local test (#917)

So that it can be copy/pasted.

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  - [PR](https://code.forgejo.org/forgejo/runner/pulls/917): <!--number 917 --><!--line 0 --><!--description Y2hvcmU6IGFkZCByZW1pbmRlciBvZiBob3cgdG8gcnVuIGEgbG9jYWwgdGVzdA==-->chore: add reminder of how to run a local test<!--description-->
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Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/917
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.code.forgejo.org>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

README.md

@@ -47,6 +47,13 @@ The [workflow](.forgejo/workflows/test.yml) that runs in the CI uses similar com
 - Install [Docker](https://docs.docker.com/engine/install/)
 - `make test integration-test`
 
+The `TestRunner_RunEvent` test suite contains most integration tests
+with real-world workflows and is time-consuming to run. During
+development, it is helpful to run a specific test through a targeted
+command such as this:
+
+- `go test -count=1 -run='TestRunner_RunEvent$/local-action-dockerfile$' ./act/runner`
+
 ### With a Forgejo instance
 
 - Run a Forgejo instance locally (for instance at http://0.0.0.0:8080) and create as shared secret