Add CI workflow (#3)

.github/workflows/ci.yml

@@ -0,0 +1,21 @@
+name: CI
+
+on: [push, pull_request]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        nim: [1.6.16, stable]
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Install Nim
+      uses: iffy/install-nim@v3
+      with:
+        version: ${{ matrix.nim }}
+    - name: Build
+      run: nimble install -y
+    - name: Test
+      run: nimble test -y