Add sample CircleCI configuration

Summary:
It appears a sample config is needed to finish configuring CircleCI.

Once enabled it can be changed to run the actual build and tests.
Closes #123

Differential Revision: D7656640

Pulled By: pietern

fbshipit-source-id: 6d9b3f60df7a3bc10c3f122a907dcde697694b94

.circleci/config.yml

@@ -0,0 +1,13 @@
+version: 2
+jobs:
+  build:
+    docker:
+      - image: debian:stretch
+    steps:
+      - checkout
+      - run:
+          name: Greeting
+          command: echo Hello, world.
+      - run:
+          name: Print the Current Time
+          command: date