added baisc usage example

example/Dockerfile

@@ -0,0 +1,2 @@
+FROM staticfloat/nginx-certbot
+COPY *.conf /etc/nginx/conf.d/

example/docker-compose.yml

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+version: '3'
+
+services:
+  proxy:
+    build: .
+    restart: always
+    environment:
+      CERTBOT_EMAIL: "your.email@example.com"
+    ports:
+      - "80:80"
+      - "443:443"

example/nginx.conf

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+server {
+    listen              443 ssl;
+    server_name         yourhostname.com;
+    ssl_certificate     /etc/letsencrypt/live/yourhostname.com/fullchain.pem;
+    ssl_certificate_key /etc/letsencrypt/live/yourhostname.com/privkey.pem;
+
+    return 200 'Let\'s Encrypt certificate successfully installed!';
+    add_header Content-Type text/plain;
+}