add CI file

.github/workflows/CI.yml

@@ -0,0 +1,24 @@
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+name: Continuous integration
+
+jobs:
+
+  fmt:
+    name: Rustfmt
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          override: true
+          profile: minimal
+          components: rustfmt
+      - uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: --all -- --check