Revise support policy for Linux distributions

[imciner2]

• Make clear that this support list is for stable 5.1
• Layout guidelines for how long each distribution is supported
• Update the versions currently under support

I will also be sending an email to the developer list asking for feedback on this vesioning proposal.

[imciner2]

After the comment period on the mailing list, I think this change in the support policy is ready to be merged.