doc: we now work with nginx cores as new as 1.13.6.

Author: agentzh

README.md

@@ -226,12 +226,12 @@ You're recommended to install this module (as well as the Nginx core and many ot
 Alternatively, you can install this module manually with the Nginx source:
 
 Grab the nginx source code from [nginx.org](http://nginx.org/), for example,
-the version 1.11.2 (see [nginx compatibility](#compatibility)), and then build the source with this module:
+the version 1.13.6 (see [nginx compatibility](#compatibility)), and then build the source with this module:
 
 ```bash
-wget 'http://nginx.org/download/nginx-1.11.2.tar.gz'
-tar -xzvf nginx-1.11.2.tar.gz
-cd nginx-1.11.2/
+wget 'http://nginx.org/download/nginx-1.13.6.tar.gz'
+tar -xzvf nginx-1.13.6.tar.gz
+cd nginx-1.13.6/
 
 Here we assume you would install you nginx under /opt/nginx/.
 ./configure --prefix=/opt/nginx \
@@ -269,6 +269,8 @@ Compatibility
 
 The following versions of Nginx should work with this module:
 
+* **1.13.x** (last tested: 1.13.6)
+* **1.12.x**
 * **1.11.x** (last tested: 1.11.2)
 * **1.10.x**
 * **1.9.x** (last tested: 1.9.15)