doc: add Meson coding style to contributors guide

doc: add Meson coding style to contributors guide

To help with consistency across all files, add a section to the
contributors guide on meson coding style. Although short, this covers
the basics for now, and can be extended in future as we see the need.

Meson style guide recommends four-space indents, like for python,
so add to editorconfig file.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>