[PR] Adding deployment to fly.io guide

[LuchoTurtle]

related to #80

This adds a guide to deploying a Flutter web app to Fly.io, using the Flutter app from https://github.com/dwyl/flutter-phoenix-channels-demo.

It does not close #80 because this just adds the deployment steps. The document detailing how to enhance the PWA is found in the following PR -> https://github.com/dwyl/flutter-phoenix-channels-demo/pull/3/

[codecov[bot]]

Codecov Report

Merging #81 (0a66cc5) into main (1ebe634) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #81   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines           70        70           
=========================================
  Hits            70        70           

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[LuchoTurtle]

This should now be ready for review. In conjunction with https://github.com/dwyl/flutter-phoenix-channels-demo/pull/3, #80 can be closed if both PRs are mergeable.