add github actions.

.github/workflows/build.yml

@@ -0,0 +1,32 @@
+name: build
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.6
+
+      - name: Build
+        shell: bash
+        run: |
+          mkdir build
+          cd build
+          cmake ..
+          make -j
+          ls -l lib
+          ls -l bin
+          ./bin/arpa_file_parser_test
+          ./bin/arpa_lm_compiler_test