[Toaster] changes to the contribute to Toaster wiki page

[Barros Pena, Belen]

I've been asked some good questions about the Toaster patch submission
process, and why it's different from the one generally followed by the
rest of the community. So I've edited the contribution instructions to
explain why things are the way they are. Additions in this section

https://wiki.yoctoproject.org/wiki/Contribute_to_Toaster#Submitting_patches

And this one

https://wiki.yoctoproject.org/wiki/Contribute_to_Toaster#Submitting_patch_s
ets_for_integration_into_Bitbake

Hopefully this makes sense.

Cheers

Belén