Merge pull request NModbus4#86 from Maxwe11/net-core

Initial support of .NET Core

Update version numbers for release 2.1.0

Add documentation and update nuspec

Add nuspec

add checking of Read() return value. without this using of non-initia…

…lized (0) value or value from the previous step of loop.

Conflicts:
	Modbus/IO/StreamResourceUtility.cs

initial commit