doc/build_system: document examples and config targets

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/build_system.txt

@@ -25,6 +25,9 @@ fate-list
 install
     Install headers, libraries and programs.
 
+examples
+    Build all examples located in doc/examples.
+
 libavformat/output-example
     Build the libavformat basic example.
 
@@ -34,6 +37,9 @@ libavcodec/api-example
 libswscale/swscale-test
     Build the swscale self-test (useful also as example).
 
+config
+    Reconfigure the project with current configuration.
+
 
 Useful standard make commands:
 make -t <target>