chore(readme): Update Xcode/node versions in README

[dpogue]

Platforms affected

iOS

Motivation and Context

README listed the incorrect Xcode version and incorrect xcworkspace file path for 8.x

Description

• Updated the Xcode and node versions
• Corrected the path to the xcworkspace file in a generated app
• Added a link to the API docs published on GitHub pages

Testing

Previewed in GitHub

Checklist

• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.86%. Comparing base (0ddab0a) to head (7cc057c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1499   +/-   ##
=======================================
  Coverage   80.86%   80.86%           
=======================================
  Files          16       16           
  Lines        1840     1840           
=======================================
  Hits         1488     1488           
  Misses        352      352           

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