Add ShellCheck CI workflow and status badges

Lint install.sh with shellcheck -S warning on every push/pull request.
Add shellcheck + GPLv3 badges to the README.

Author: drorganvidez

.github/workflows/shellcheck.yml

@@ -0,0 +1,19 @@
+name: shellcheck
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  shellcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Ensure ShellCheck is available
+        run: shellcheck --version || (sudo apt-get update && sudo apt-get install -y shellcheck)
+      - name: Lint install.sh
+        run: shellcheck -S warning install.sh

README.md

@@ -1,5 +1,8 @@
 # linuxroam
 
+[![shellcheck](https://github.com/diverso-lab/linuxroam/actions/workflows/shellcheck.yml/badge.svg)](https://github.com/diverso-lab/linuxroam/actions/workflows/shellcheck.yml)
+[![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](LICENSE)
+
 Clean installer to join the **eduroam** Wi-Fi network on Linux with NetworkManager.
 Works for **any institution** registered in the eduroam CAT database (most universities and research centers worldwide).