docs: create v2 Migration document

[jonathanong]

closes #920

[codecov]

Codecov Report

Merging #931 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #931   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         425    425           
  Branches      101    101           
=====================================
  Hits          425    425

Impacted Files	Coverage Δ
lib/application.js	100% <ø> (ø)	✅

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