Add a note about compilation

tests/README

@@ -1,3 +1,11 @@
+Compilation
+-----------
+
+After installation, you can use pkg-config to compile these tests.
+For example, to compile random-test-slave run:
+
+gcc random-test-slave.c -o random-test-slave `pkg-config --libs --cflags modbus`
+
 random-test-slave
 -----------------
 It's necessary to launch this server before run random-test-master. By