updated "Accessing External Build process' logs" section

Since IDEA 14.1 logging is configured via properties file, not xml file.

reference_guide/frameworks_and_external_apis/external_builder_api.md

@@ -93,10 +93,12 @@ The log file is located under the directory
 <ide-system-directory>/log/build-log
 ```
 
-There both `build-log.log` and `build-log.xml` files can be found.
-The `build-log.xml` is a log4j configuration file, where the log level and desired logging categories can be adjusted.
+There both `build-log.log` and `build-log.properties` files can be found.
+The `build-log.properties` is a log4j configuration file, where the log level and desired logging categories can be adjusted. 
 This file contains logging from all  build sessions, including those from the auto-make.
 
+In IntelliJ IDEA versions before version 14.1 log4j configuration was stored in `build-log.xml`.
+
 ### Accessing project model and configuration from External Build
 
 The project model in External Build process is provided by JPS (*JetBrains Project System*).