Add instructions regarding how to build and test xfce4-netload-plugin

Hello!

I just did:

$ ./autogen.sh
$ make -j4

It created panel-plugin/libnetload.la but

1. How do I test it with xfce4 panel without overwriting the distro provided libnetload.la?
2. How do I build executable for panel-plugin/commandline.c? I believe it's for CLI testing?

These instructions should be in README or in a separate file e.g. HACKING.

Thanks!