Move UPGRADE to UPGRADE.md

Descriptive title for this enhancement

To make it easier to read the contrib/docs/UPGRADE should be moved to the root of the source and be converted into markdown UPGRADE.md

Actual behavior

UPGRADE is a file containing all the upgrade step from one version to another, its in a special directory and not directly accessible. Also its in pure text and would benefit from a markdown conversion.

Expected behavior

Have a UPGRADE.md in the root of the sources and make it easy for people to see whats has to be done in each upgrade steps

Benefits

Easier to read doc, more people will be able to see it