Update CI workflow to include pull requests (#33)

Previously CI would run only on pushes to your own repo.  With this change, it will work for PRs from forks too.

Author: asomers

.github/workflows/vmactions.yml

@@ -1,6 +1,9 @@
 name: CI
 
-on: [push]
+on:
+  push:
+    branches: [main, master]
+  pull_request:
 
 env:
   CARGO_TERM_COLOR: always