Updating spec file to work with haproxy 3.2.x

[sharumpe]

When attempting a build of haproxy 3.2.4, I got errors about a couple of missing files:

• README (which is now README.md in 3.2.0 and above)
• docs/architecture.txt (which does not exist in 3.2.0 and above)

I don't know enough about how spec files may be able to use different options if, for example, the version is 3.2 or above. That would probably make most of this unnecessary, but for now, this is what I've had success with.

This pull request contains two changes:

• Updating Makefile to allow specifying a different spec file with USE_SPEC=<filename>
• Adding duplicate specfile haproxy3.spec, which has fixed references in the %doc section

My build process now specifies USE_SPEC=haproxy3.spec with the other options, which allowed the build to succeed.

[sharumpe]

OK, when I asked it to make a draft pull request, it worked. I'll reference this from issue #71 . Apologies for the overlap.