[ticket/12776] Add section about 3.0.x to 3.1.x update to INSTALL.html

PHPBB3-12776
Merge remote-tracking branch 'github-nickvergessen/ticket/12776' into develop-ascraeus

* github-nickvergessen/ticket/12776:
  [ticket/12776] Add section about 3.0.x to 3.1.x update to INSTALL.html