Add CodeQL scan

.github/workflows/codeql-analysis.yml

@@ -0,0 +1,32 @@
+name: "CodeQL scan"
+
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: '0 16 * * 2'
+
+jobs:
+  CodeQL-Build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 2
+
+    - run: git checkout HEAD^2
+      if: ${{ github.event_name == 'pull_request' }}
+
+    - run: sudo apt-get install -qy libsodium-dev
+      
+    - name: Initialize CodeQL
+      uses: github/codeql-action/init@v1
+
+    - name: Autobuild
+      uses: github/codeql-action/autobuild@v1
+      
+    - name: Perform CodeQL Analysis
+      uses: github/codeql-action/analyze@v1

README.md

@@ -1,3 +1,4 @@
+![CodeQL scan](https://github.com/jedisct1/minisign/workflows/CodeQL%20scan/badge.svg)
 
 Minisign
 ========