Path for pkg-config

See #497, bottom comment.

Author: cdunn2001

Diff for: README.md

@@ -97,6 +97,10 @@ Alternatively, from the command-line on Unix in the source directory:
     cmake -DCMAKE_BUILD_TYPE=debug -DBUILD_STATIC_LIBS=ON -DBUILD_SHARED_LIBS=OFF -DARCHIVE_INSTALL_DIR=. -G "Unix Makefiles" ../..
     make
 
+For a good pkg-config file, add:
+
+    -DCMAKE_INSTALL_INCLUDEDIR=include/jsoncpp
+
 Running `cmake -h` will display the list of available generators (passed using
 the `-G` option).