Add Github build action.

.github/workflows/build.yml

@@ -0,0 +1,28 @@
+name: Build randopix
+# This workflow is triggered on pushes to the repository.
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  build:
+    name: Build x86
+    # This job runs on Linux
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: Cache choosenim
+      id: cache-choosenim
+      uses: actions/cache@v1
+      with:
+        path: ~/.choosenim
+        key: ${{ runner.os }}-choosenim-stable
+    - name: Cache nimble
+      id: cache-nimble
+      uses: actions/cache@v1
+      with:
+        path: ~/.nimble
+        key: ${{ runner.os }}-nimble-stable
+    - uses: jiro4989/setup-nim-action@v1.0.2
+    - run: nimble build -Y