added example to readme

Author: burner1024

README.md

@@ -1,3 +1,28 @@
 ## Jenkins Job Builder action
 
 This action launches [Jenkins Job Builder](https://jenkins-job-builder.readthedocs.io/en/latest/) to update your Jenkins jobs.
+
+## Example
+
+```yaml
+name: jjb
+
+on: [push]
+
+jobs:
+  jjb:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: devopsx/gha-jjb@master
+        with:
+          jenkins_token: ${{ secrets.JENKINS_TOKEN }}
+          jjb_yml: jenkins/jobs.yml
+          jjb_ini: jenkins/jenkins_jobs.ini
+```
+
+## Configuration
+
+1. Create Jenkins [API token](https://www.jenkins.io/blog/2018/07/02/new-api-token-system/).
+2. Create `JENKINS_TOKEN` [secret](https://docs.github.com/en/actions/reference/encrypted-secrets#creating-encrypted-secrets-for-a-repository) in repo settings.
+3. Add gha yaml, as described above. That's it!