update .env.example to include SCAN_CACHE_TTL for directory scan caching

.env.example

@@ -4,4 +4,8 @@ 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
+FLASK_DEBUG=0
+
+# Seconds to cache the TeslaCam directory scan (default 300). Hit /refresh to
+# force a re-scan.
+SCAN_CACHE_TTL=3600