Update readme with fix

[coenm]

Update README with additional information how to use multiple expressions.

According to dotnet/msbuild#2999 (comment) you'll have to escape the surrounding quotes in a parameter with a comma.

This has been described in the readme.

An other fix would be to have an other sperator. I.e. a space (like opencover).

[codecov]

Codecov Report

Merging #100 into master will decrease coverage by 12.33%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #100       +/-   ##
===========================================
- Coverage   98.91%   86.57%   -12.34%     
===========================================
  Files          15       15               
  Lines        1378     1378               
===========================================
- Hits         1363     1193      -170     
- Misses         15      185      +170

Impacted Files	Coverage Δ
src/coverlet.core/Coverage.cs	14.13% <0%> (-85.87%)	⬇️
src/coverlet.core/Helpers/InstrumentationHelper.cs	91.32% <0%> (-6.94%)	⬇️

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