Added instructions on using Swift plugin in ObjC App

[mvanbeusekom]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Docs update

⤵️ What is the current behavior?

There are some problems when adding a Swift plugin into a Objective-C App. Instructions on how to workaround this problem are missing.

🆕 What is the new behavior (if this is a feature change)?

Added instructions on how to work around the problem of using a Swift plugin in a Objective-C App.

💥 Does this PR introduce a breaking change?

No

🐛 Recommendations for testing

n/a

📝 Links to relevant issues/docs

Flutter#16049

🤔 Checklist before submitting

• All projects build
• Rebased onto current develop

[spockfish]

De zin loopt niet helemaal.

There's a known issue with integrating plugins that use Swift.

[codecov-io]

Codecov Report

Merging #19 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #19   +/-   ##
========================================
  Coverage    93.93%   93.93%           
========================================
  Files            2        2           
  Lines           33       33           
========================================
  Hits            31       31           
  Misses           2        2

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