Make 2.0.0 release

[kghbln]

Tracking issue for the 2.0.0 release.

• Release notes
• Changes since 1.2.0

• Finalize RELEASE NOTES
• Update "README.md"
• Update "composer.json"
• Update version number "extension.json"
• Create tag
• Add release to Wikidata
• Update homepage
• Announce via mail
• Do a release tweet on Twitter
• Run composer update on smw.o
• Update "LocalSettings.php"
• Start development of new version
  • Update "composer.json" (update dev alias)
  • Update "extension.json" (new alpha version)

[kghbln]

@mwjames Now that #38 is finally done, hope so at least, I'd like to move this out to the public