Add optional standalone executable directory (#2)

* add standalone executable

* update readme

* add standalone workflow

* fix path

.github/workflows/standalone.yml

@@ -0,0 +1,20 @@
+name: Standalone
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    
+    steps:
+    - uses: actions/checkout@v1
+    
+    - name: configure
+      run: cmake -Hstandalone -Bbuild
+
+    - name: build
+      run: cmake --build build
+
+    - name: run
+      run: ./build/Greeter