Heroku manifest updated with environment vars

Author: osminogin

app.json

@@ -3,5 +3,12 @@
   "description": "Self-destructing notes written on Go",
   "repository": "https://github.com/osminogin/tornote",
   "keywords": ["go", "golang"],
-  "addons": ["heroku-postgresql:hobby-dev"]
+  "addons": ["heroku-postgresql:hobby-dev"],
+  "env": {
+    "HTTPS_ONLY": "False",
+    "SECRET_KEY": {
+      "description": "A secret key for a particular installation.",
+      "generator": "secret"
+    }
+  }
 }