add .env.example and update README for environment variable configuration

.env.example

@@ -0,0 +1,7 @@
+# Root TeslaCam directory containing RecentClips/SavedClips/SentryClips
+TESLACAM_DIRECTORY=/mnt/TeslaCam
+
+# Local dev server only (ignored when run under a WSGI server like gunicorn)
+FLASK_RUN_HOST=0.0.0.0
+FLASK_RUN_PORT=5000
+FLASK_DEBUG=0