Add link to last source of main packages
As show in https://discuss.tryton.org/t/trouble-installing-tryton-web-client/4942/8 it is not clear enough that the source files browse contains all the sources to download.
As show in https://discuss.tryton.org/t/trouble-installing-tryton-web-client/4942/8 it is not clear enough that the source files browse contains all the sources to download.