Document end of life policy on axe-core

Deque is discussing a policy on how far back Axe-core will be supported in other products such as Axe Extension. It comes prepackages with them, and is therefor limited in how many we can have. We're likely looking at all minor versions released in the least 15 months. We'll want to follow that policy for security vulnerabilities as well. If any security vulnerability is discovered, we'll go back and patch it for all minor versions released in the last 15 months.

We'll need to put the info in https://github.com/dequelabs/axe-core/blob/develop/doc/release-and-support.md once the policy is decided on.