shellcheck: add a github shellcheck workflow

We want to ensure the script remains clean over time
 and evolutions. Shellcheck is a tool that point out 
many possible mistakes in shell scripts to avoid bugs.

Create a github workflow to analyse the script each time a commit is pushed.

.github/workflows/blank.yml

@@ -0,0 +1,12 @@
+name: CI
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: shellcheck
+    - uses: ludeeus/[email protected]