NMLv2.1

What's Changed

• Add validation instructions to readme by @sanjayankur31 in #131
• Experimental by @pgleeson in #132
• generate.py: Fix for python39 and cosmetic tweaks by @sanjayankur31 in #133
• fix(generate.py): use Decimal to correctly report conversion factors by @sanjayankur31 in #135
• chore: remove extra space around "_" by @sanjayankur31 in #141
• For NML v2.1 release by @pgleeson in #143

Full Changelog: NMLv2.0...NMLv2.1