Accept and move to rust2rpm.conf as standard config file location
Using .rust2rpm.conf is confusing to new users, because they never see
these files in existing packages and are only seeing magic happen.
This commit changes the default location of the configuration file to
./rust2rpm.conf, and will rename existing .rust2rpm.conf or _rust2rpm.conf
files to the new name if they already exist.
I have also added a little more documentation about the config file itself
to the README.