Create build.yml

This adds GithubAction that  runs the build and checks for any unstaged
changes. This ensures `src/dir-index.html` match the output `dir-index.html`
in the main directory.

.github/workflows/build.yml

+name: Build
+on:
+  push:
+
+jobs:
+  no-uncommitted-changes:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        ref: ${{ github.head_ref }}
+    - name: Run Build
+      uses: actions/setup-node@v1
+      with:
+        node-version: '12.x'
+    - run: npm run build    
+    - name: Check working tree
+      id: changes
+      uses: UnicornGlobal/has-changes-action@v1.0.11
+    - name: Check for uncommited changes
+      if: steps.changes.outputs.changed == 1
+      run: git status && echo "Output does not match src/, uncommitted changes exist, follow build instructions from README" && exit 1