Add not to README.md about the location names

1 files changed, 2 insertions, 0 deletions

diff --git a/README.md b/README.md
index 7bbd52d..2e5f4fc 100644
--- a/README.md
+++ b/README.md
@@ -93,6 +93,8 @@ This _config.yml_ also contains the credentials for an updated geoip database. T
 
 Then edit _group_vars/all/gateway_locations.yml_, _group_vars/all/provider_config.yml_ to match your environment. 
 
+NOTE: The value of `location` for a VPN gateway host, and the location keys in _gateway_locations.yml_, **must** match exactly one of [these city names](https://github.com/tidwall/cities/blob/master/cities.go).
+
 ## 4. Generate credentials 
 
 ... by running the init-credentials playbook. This will ansible-vault-encrypt the resulting secret files under _credentials/_.