Documentation: how to write plugins for Grilo

[Iago Toral Quiroga <itoral igalia com>]

Hi,

I have just pushed an update to our documentation, adding a chapter on
how to write plugins for Grilo. Check it out and let me know if you have
any suggestions [1].

Obviously, some of the proposals we are considering for the short term,
like the GRLOptions APIs, would make this documentation obsolete to some
extent, so let's have in mind that we have to update the docs when we go
forward with this type of changes from now on.

[1] Build grilo with --enable-gtk-doc and then point your browser to
doc/grilo/html/index.html.

Iago