issue-280: change to using cppalliance for appveyor

[jeking3]

Personal accounts have one build runner, for example:

https://ci.appveyor.com/project/Flamefire/boost-ci/builds/52349304

I enabled boost-ci on the cpp-alliance appveyor account which has 4 runners, so builds will go faster. The only change needed here is updating the README, and @Flamefire can disable and delete the boostorg/boost-ci appveyor connection in his account.

Curiously, I only changed README.md and yet is still built, even with paths-ignore.

This fixes #280

[Flamefire]

Curiously, I only changed README.md and yet is still built, even with paths-ignore.

I can't see any appveyor build running for this PR, so I'd guess it works.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (a0ec925) to head (49c02f7).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #291   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           22        22           
  Branches        12        12           
=========================================
  Hits            22        22           

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[Flamefire]

I deleted the boost-ci Appveyor project from my account now