Add a build badge to readme

.github/workflows/build.yml

@@ -0,0 +1,18 @@
+name: build
+
+on:
+  - push
+  - pull_request
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Set up Go
+        uses: actions/setup-go@v2
+        with:
+          go-version: 1.14
+      - name: Run go test
+        run: go test ./... -v