Extend documentation about auto-build-requires

[mizdebsk]

Although there are some mentions of xmvn-builddep and Mock pm_request, but I perhaps that this may be insufficient.

IMHO, builddep is a great feature, which has been available for almost 4 years. Together with Mock pm_request plugin it effectively enables auto-build-requires. Unfortunately I have a feeling that it is not used widely and often enough.