Update README: Add note about ts in textfile
This closes
@@ -92,7 +92,7 @@ that are tied to a machine.
To use it, set the `--collector.textfile.directory` flag on the Node exporter. The
collector will parse all files in that directory matching the glob `*.prom`
using the [text
-format](http://prometheus.io/docs/instrumenting/exposition_formats/).
+format](http://prometheus.io/docs/instrumenting/exposition_formats/). **Note:** Timestamps are not supported.
To atomically push completion time for a cron job:
```
