Add notes on testing with act

CONTRIBUTING.md

@@ -1,4 +1,4 @@
-### How to merge a Dependabot PR
+## How to merge a Dependabot PR
 
 The "distribution" for a GitHub Action is checked into the repository itself. 
 In the case of these actions, the transpiled sources are committed to the `dist` directory. 
@@ -12,3 +12,27 @@ The simplest process to follow is:
 3. In the `sources` directory, run `npm run build` to regenerate the distribution
 4. Push the changes to the dependabot branch
 5. If/when the checks pass, you can merge the dependabot PR
+
+## Using `act` to run integ-test workflows locally
+
+It's possible to run GitHub Actions workflows locally with https://nektosact.com/.
+Many of the test workflows from this repository can be run in this way, making it easier to
+test local changes without pushing to a branch.
+
+This feature is most useful to run a single `integ-test-*` workflow. Avoid running `ci-quick-test` or other aggregating workflows unless you want to use your local machine as a heater!
+
+Example running a single workflow:
+`act -W .github/workflows/integ-test-caching-config.yml`
+
+Example running a single job:
+`act -W .github/workflows/integ-test-caching-config.yml -j cache-disabled-pre-existing-gradle-home`
+
+Known issues:
+- `integ-test-cache-cleanup.yml` fails because `gradle` is not installed on the runner. Should be fixed by #33.
+- `integ-test-detect-java-toolchains.yml` fails when running on a `linux/amd64` container, since the expected pre-installed JDKs are not present. Should be fixed by #89.
+
+Tips:
+- Add the following lines to `~/.actrc`:
+    - `--container-daemon-socket -` : Prevents "error while creating mount source path", and yes that's a solitary dash at the end
+    - `--matrix os:ubuntu-latest` : Avoids a lot of logging about unsupported runners being skipped
+- Runners don't have `java` installed by default, so all workflows that run Gradle require a `setup-java` step.