* Change the directory location and update readme

When conan is used to build the dependencies (orca & xerces) of gpdb, it
copies the headers and libraries to the path specified in
the imports section of the conanfile.txt.
Changing the target copy location to /usr/local/include and
/usr/local/lib as its the default for gpdb. In case the user
prefers to have a different directory, they can change the location
accordingly.