chore: update readme for github resource detector

Update README of GitHub Resource Detector to clarify the source of the data.

Part Of #2055

Signed-off-by: maryliag <marylia.gutierrez@grafana.com>

detectors/node/opentelemetry-resource-detector-github/README.md

@@ -33,6 +33,21 @@ async function run() {
 run()
 ```
 
+## Available detectors
+
+### GitHub Detector
+
+| Attribute         | Description                                               |
+|-------------------|-----------------------------------------------------------|
+| github.actor      | Value of Process Environment Variable `GITHUB_ACTOR`      |
+| github.base_ref   | Value of Process Environment Variable `GITHUB_BASE_REF`   |
+| github.head_ref   | Value of Process Environment Variable `GITHUB_HEAD_REF`   |
+| github.ref        | Value of Process Environment Variable `GITHUB_REF`        |
+| github.run_id     | Value of Process Environment Variable `GITHUB_RUN_ID`     |
+| github.run_number | Value of Process Environment Variable `GITHUB_RUN_NUMBER` |
+| github.sha        | Value of Process Environment Variable `GITHUB_SHA`        |
+| github.workflow   | Value of Process Environment Variable `GITHUB_WORKFLOW`   |
+
 ## Useful links
 
 - [GitHub Action Environment Variables](https://docs.github.com/en/free-pro-team@latest/actions/reference/environment-variables)