RPM Packaging Workspace: add details on BUILDROOT directory

[nqb]

Hello,

I suggest a note in following section https://github.com/redhat-developer/rpm-packaging-guide/blob/master/source/packaging-software.adoc#rpm-packaging-workspace about BUILDROOT directory.

When you first run rpmdev-setuptree directory isn't created but after running your first rpmbuild command, you will get a BUILDROOT directory.

It will be nice to add a description about this directory.