Add GitHub CI.

.github/workflows/ci.yaml

@@ -0,0 +1,39 @@
+name: CI
+on: [push]
+
+jobs:
+  build-linux:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - name: Build
+        run: cargo build --verbose
+      - name: Test
+        run: cargo test --verbose
+
+  build-windows:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v1
+      - name: Build
+        run: cargo build --verbose
+      - name: Test
+        run: cargo test --verbose
+
+  build-macosx:
+    runs-on: macosx-latest
+    steps:
+      - uses: actions/checkout@v1
+      - name: Build
+        run: cargo build --verbose
+      - name: Test
+        run: cargo test --verbose
+
+  check-readme:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - name: Install cargo-sync-readme
+        run: cargo install --force cargo-sync-readme
+      - name: Check
+        run: cargo sync-readme -c