Update README of cpp-api

Author: cyang-kth

example/cpp-api/README.md

@@ -5,7 +5,7 @@ Build the program
 ```
 mkdir build
 cd build
-cmake .. -DFMM_LIB -DFMM_HEADER
+cmake ..
 make
 sudo make install
 ```
@@ -15,3 +15,16 @@ Run it with
 ```
 fmm_print_network example/data/edges.shp
 ```
+
+You should see output like
+
+```
+[2020-12-04 13:50:29.350] [info] [main.cpp:9] Use FMM in an external project
+[2020-12-04 13:50:29.350] [info] [main.cpp:14] Network file example/data/edges.shp
+[2020-12-04 13:50:29.351] [info] [network.cpp:72] Read network from file example/data/edges.shp
+[2020-12-04 13:50:29.356] [info] [network.cpp:166] Number of edges 30 nodes 17
+[2020-12-04 13:50:29.356] [info] [network.cpp:168] Field index: id 1 source 2 target 3
+[2020-12-04 13:50:29.356] [info] [network.cpp:170] Read network done.
+[2020-12-04 13:50:29.356] [info] [main.cpp:16] Network node count 17
+[2020-12-04 13:50:29.356] [info] [main.cpp:17] Network edge count 30
+```