Fix and clarify instructions for building address book example.

c++/samples/addressbook.c++

@@ -21,9 +21,14 @@
 
 // This sample code appears in the documentation for the C++ implementation.
 //
-// Compile with:
-//   capnpc -oc++ addressbook.capnp
-//   c++ -std=c++11 -Wall addressbook.c++ addressbook.capnp.c++ -lcapnp -o addressbook
+// If Cap'n Proto is installed, build the sample like:
+//   capnp compile -oc++ addressbook.capnp
+//   c++ -std=c++11 -Wall addressbook.c++ addressbook.capnp.c++ `pkg-config --cflags --libs capnp` -o addressbook
+//
+// If Cap'n Proto is not installed, but the source is located at $SRC and has been
+// compiled in $BUILD (often both are simply ".." from here), you can do:
+//   $BUILD/capnp compile -I$SRC/src -o$BUILD/capnpc-c++ addressbook.capnp
+//   c++ -std=c++11 -Wall addressbook.c++ addressbook.capnp.c++ -I$SRC/src -L$BUILD/.libs -lcapnp -lkj -o addressbook
 //
 // Run like:
 //   ./addressbook write | ./addressbook read