add github action to ensure that ECP-ST-CAR always builds

This adds a [github action](https://help.github.com/en/articles/workflow-syntax-for-github-actions)
that builds the document on every pull request. The goal is to ensure that it builds before merging.

Author: tgamblin

.github/workflows/test.yml

@@ -0,0 +1,17 @@
+# documentation: https://help.github.com/en/articles/workflow-syntax-for-github-actions
+name: build document
+on: [push, pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - name: checkout
+      uses: actions/checkout@v2
+    - name: install prerequisites
+      run: sudo apt-get install make
+    - name: Compile LaTeX document
+      uses: xu-cheng/latex-action@v2
+      with:
+        root_file: ECP-ST-CAR.tex
+        compiler: make
+        args: