License fixes

[spike77453]

I've recently created a mkrdns rpm package for Fedora. The Fedora Licensing Guidelines suggest that

If the source package does not include the text of the license(s), the packager should contact upstream and encourage them to correct this mistake.

This PR

• Adds the GNU GPL v2.0 license file from https://www.gnu.org/licenses/old-licenses/gpl-2.0.md
• Updates the licensing information in the README.md
• Fixes the FSF's address

so that this repository complies with https://www.gnu.org/licenses/old-licenses/gpl-2.0.html and the Fedora Licensing Guidelines.

I'd appreciate it if you'd accept these suggestions.

[jdkelleher]

mkrdns.dist should be reverted to remain identical to v3.3 found here - https://sites.google.com/a/kluge.net/mkrdns/

[oasys]

I agree with @jdkelleher about keeping mkrdns.dist unmodified.

@spike77453, if you update the PR, I'd be happy to merge.

[spike77453]

I've updated to commit in question to leave mkrdns.dist unmodified.