Create README.txt

Author: josesimoes

source/nanoFramework.System.Net/README.txt

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+After updating the Nuget package(s)
+
+1. After updating the Nuget packages, the project file (nfproj extension) has to be manually edited too in order to update the path for the new package for all <NFMDP_PE_LoadHints> tags.
+
+2. The nuspec files  have to be manually updated for the dependency versions to match the referenced packages.