README.md - mention WPA_PASSWORD length requirement

Author: Andrew Pattison

README.md

@@ -119,7 +119,7 @@ The following environment variables are supported:
 
  * `WPA_ESSID`, `WPA_PASSWORD` and `WPA_COUNTRY` (Default: unset)
 
-   If these are set, they are use to configure `wpa_supplicant.conf`, so that the raspberry pi can automatically connect to a wifi network on first boot. If `WPA_ESSID` is set and `WPA_PASSWORD` is unset an unprotected wifi network will be configured.
+   If these are set, they are use to configure `wpa_supplicant.conf`, so that the raspberry pi can automatically connect to a wifi network on first boot. If `WPA_ESSID` is set and `WPA_PASSWORD` is unset an unprotected wifi network will be configured. If set, `WPA_PASSWORD` must be between 8 and 63 characters.
 
  * `ENABLE_SSH` (Default: `0`)