Add info on whitelisting (home-assistant#5197)

Needs to be added as causing issues for users https://community.home-assistant.io/t/0-67-whitelisted-dir-issues/50305/5

Author: robmarkcole

source/_components/sensor.file.markdown

@@ -14,7 +14,7 @@ ha_release: 0.45
 ---
 
 
-The `file` sensor platform reads the entries from a plain-text file and shows the found value. Only the last line of the file is used. This is similar to do `$ tail -n 1 sensor.txt` on the command-line.
+The `file` sensor platform reads the entries from a plain-text file and shows the found value. Only the last line of the file is used. This is similar to do `$ tail -n 1 sensor.txt` on the command-line. Note that file paths must be added to [whitelist_external_dirs](/docs/configuration/basic/).
 
 To enable the `file` sensor, add the following lines to your `configuration.yaml`: