Add explicit permissions to GitHub Actions workflow (#19)

Resolves CodeQL security alert by restricting GITHUB_TOKEN to contents:read only

Author: mraible

.github/workflows/main.yml

@@ -1,6 +1,9 @@
 name: Sample CI
 on: [ push, pull_request ]
 
+permissions:
+  contents: read
+
 jobs:
   build:
     name: Build and Test