Add README to NuGet package

[sys27]

https://devblogs.microsoft.com/nuget/add-a-readme-to-your-nuget-package/

Affected By: NuGet/Home#10791

[codecov]

Codecov Report

Merging #423 (a3020ee) into dev (b334fd9) will decrease coverage by 0.03%.
The diff coverage is n/a.

❗ Current head a3020ee differs from pull request most recent head 1e8b67e. Consider uploading reports for the commit 1e8b67e to get more accurate results

@@            Coverage Diff             @@
##              dev     #423      +/-   ##
==========================================
- Coverage   99.42%   99.39%   -0.04%     
==========================================
  Files         174      168       -6     
  Lines        6253     6072     -181     
==========================================
- Hits         6217     6035     -182     
- Misses         36       37       +1     

Impacted Files	Coverage Δ
xFunc.Maths/Expressions/Units/Unit.cs	96.15% <0.00%> (-3.85%)	⬇️
xFunc.Maths/Expressions/LogicalAndBitwise/Bool.cs	93.10% <0.00%> (-3.33%)	⬇️
xFunc.Maths/Expressions/Number.cs	96.77% <0.00%> (-3.23%)	⬇️
...nc.Maths/Expressions/Collections/ParameterValue.cs	97.05% <0.00%> (-0.24%)	⬇️
xFunc.Maths/Processor.cs	98.52% <0.00%> (-0.12%)	⬇️
...Func.Maths/Analyzers/TypeAnalyzers/TypeAnalyzer.cs	99.87% <0.00%> (-0.01%)	⬇️
xFunc.Maths/Parser.cs	100.00% <0.00%> (ø)
xFunc.Maths/Builder.cs	100.00% <0.00%> (ø)
xFunc.Maths/Helpers.cs	91.48% <0.00%> (ø)
xFunc.Maths/ThrowHelpers.cs	100.00% <0.00%> (ø)
... and 164 more

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