added docu for the letsencrypt backend

README.md

@@ -57,6 +57,22 @@ san: []
 
 ### Backends
 #### Letsencrypt
+
+*letsencrypt-backend-config*
+
+```
+# days of validity left on a certificate bevore it is renewed
+remainingdays: 28
+
+# challange type to use, can be:
+# 'dns-01': use the dns challange and a custom power dns backend
+# 'http-01: use the http challange and deploy the challanges to a webserver
+challange: dns-01
+
+# servers to deploy a challange to
+challangeserver: []
+```
+
 #### Selfsigned
 
 ## Paths