Add note to avoid "not found rpm-py-installer" in project provided as a RPM.

[junaruga]

When a project implements rpm-py-installer as a required dependency, then later tried to release the project as a RPM package, the project have to consider rpm-py-installer is not provided as a RPM package.

Maybe RPM spec file would consider that a required dependency package exists as a RPM package.
But I have no plan to provide rpm-py-installer as a RPM package.

The solution that a python project of this case did is here to consider the status in setup.py.
https://github.com/rebase-helper/rebase-helper/blob/master/setup.py

Here is a issue that rpkg faced.
https://bugzilla.redhat.com/show_bug.cgi?id=1579367
And the possible solution.
https://pagure.io/rpkg/pull-request/319

I think I have to write this on README for people to know this issue in advance.
Providing best practices to fix it on README might be better.

ex.

rpm_spec_file_of_your_project.spec

%prep
...
sed -i '/rpm-py-installer/d' setup.py
...

or

%prep
...
sed -i 's/rpm-py-installer/rpm/' setup.py
...