updated README

README.md

 Configuration
 -------------
 
-Configuration file: `/etc/letsencrypt/certbot-pdns.json`:
+An example file is provided in `/usr/local/etc/letsencrypt/certbot-pdns.json`:
+```
+{
+  "api-key": "change_it",
+  "base-url": "http://127.0.0.1:34022/api/v1",
+  "axfr-time": 5
+}
+```
+
+Configuration file must be placed in `/etc/letsencrypt/certbot-pdns.json` or be specified with argument `certbot-pdns-config`.
+
+Configuration keys:
+
  - api-key: Your PowerDNS API Key as specified in property `api-key` in file `/etc/powerdns/pdns.conf`
  - base-url: The base URL for PowerDNS API. Require `api=yes` and `api-readonly=no` in file `/etc/powerdns/pdns.conf`
- - axfr-time: The time in seconds to wait for zone replication in slaves. Can be set to 0 if there is only one authoritative server for the zone.
+ - axfr-time: The time in seconds to wait for AXFR in slaves. Can be set to 0 if there is only one authoritative server for the zone.
 
 Usage
 -----
 
-Use certbot as usual but specify `--authenticator certbot-pdns:auth certonly`:
+Use certbot as usual but specify `--authenticator certbot-pdns:auth`:
 
 `certbot --agree-tos --text --renew-by-default --authenticator certbot-pdns:auth certonly -d example.com -d www.example.com`