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

• [x] I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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