Code Coverage scripts, CI, README.md

[gfoidl]

https://codecov.io/gh/gfoidl/Base64

[gfoidl]

Scripts, etc. are taken over from gfoidl-Tests/Continuous-Integration-Test@eeb5518

[gfoidl]

OSX-build shows failures:

A fatal error occurred, the required library libhostfxr.dylib could not be found.
If this is a self-contained application, that library should exist in [/Users/vsts/.dotnet/tools/.store/coverlet.console/1.3.0/coverlet.console/1.3.0/tools/netcoreapp2.1/any/].
If this is a framework-dependent application, install the runtime in the default location [/usr/local/share/dotnet] or use the DOTNET_ROOT environment variable to specify the runtime location.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@81b736b). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #28   +/-   ##
=========================================
  Coverage          ?   98.87%           
=========================================
  Files             ?       11           
  Lines             ?     1964           
  Branches          ?        0           
=========================================
  Hits              ?     1942           
  Misses            ?       22           
  Partials          ?        0

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[gfoidl]

Windows-Debug-build fails:

Unable to read beyond the end of the stream.

This is a known bug coverlet-coverage/coverlet#210
So as workaround temporarily disable coverage on win-debug-build.