New Optimization Section on d.g.o

[Callum <callum physics otago ac nz>]

As a result of last weeks memory optimization discussion, I have added a
new "Optimization" section to d.g.o under the Programming Guides section.

Currently "Optimizing GNOME Software" contains a single article on using
Massif for memory profiling. I hope to be able to get a couple more
articles up myself, but further contributions (and criticism of the
existing article) will be very welcome.

 - Callum