WIP: README.md drop support for Xcode pre-9.0 - WIP FOR DISCUSSION

[brodycj]

Platforms affected

iOS

What does this PR do?

• Drop support for Xcode pre-9.0 in README.md
• add notice that Xcode 10 (beta) is not supported, with known issue ref:
  • https://issues.apache.org/jira/browse/CB-14192
  • [CB-14192] Xcode 10 issue notice cordova-docs#862
  • CB-14192: (ios) Add project settings to explicitly use legacy build s… #378 (proposed workaround solution for Xcode 10)

TODO:

• needs discussion
• raise JIRA issue if agreed
• update in cordova-docs
• check if we should drop support for Xcode pre-9.0 in bin and tests?

What testing has been done on this change?

TODO

Checklist

TODO:

• Reported an issue in the JIRA database
• Commit message follows the format: "CB-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
• Added automated test coverage as appropriate for this change.

[codecov-io]

Codecov Report

Merging #386 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #386   +/-   ##
=======================================
  Coverage   65.62%   65.62%           
=======================================
  Files          14       14           
  Lines        1702     1702           
  Branches      286      286           
=======================================
  Hits         1117     1117           
  Misses        585      585

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[jcesarmobile]

We can probably drop Xcode 9 too as Xcode 10 will be mandatory starting in March (but only for App Store submissions, some people might still want to use Xcode 9 for enterprise apps)

[brodycj]

I am closing this one in favor of new PR #491. I would favor documenting Xcode 9 support as "deprecated", as I proposed in the new PR (#491).