Changelog link broken on docs.rs site

[dmackinn]

Summary

When you click the changelog link under the Versions section at https://docs.rs/crate/rand/latest you receive an error page with the following

Cannot parse versionwith valueCHANGELOG.md: unexpected character 'C' while parsing major version number

[newpavlov]

It's because we use a relative link (same with "Upgrade Guide"). It's probably worth to replace it with an absolute link to GitHub.

[dhardy]

This is a simple tweak to the README. No new CHANGELOG entry needed. PR please.