Version 4.5.1.Added small readme file for .Net Core sample project

SampleApp.Core/readme.txt

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+EPPlus samples for .Net Core
+
+Solution can be opened in Visual Studio for Windows or MacOS.
+On other operation systems please use...
+
+dotnet restore
+dotnet run
+
+... to execute sample. Non-windows operationsystems will requires libgdiplus to be installed. 
+Please use your favorite package manager to install it. 
+For example:
+
+Homebrew on MacOS:
+brew install mono-libgdiplus
+
+apt-get:
+apt-get install libgdiplus
+
+Also see wiki on https://github.com/JanKallman/EPPlus/wiki for more details