add ci, silence invalid warning

.github/workflows/main.yml

@@ -0,0 +1,9 @@
+name: Shellcheck
+on: [push, pull_request]
+jobs:
+    build:
+        runs-on: ubuntu-latest
+        steps:
+            - uses: actions/checkout@v1
+            - name: Run shellcheck.
+              run: shellcheck shfm