CPP:Update README on ICU version. Patch contributed by roubert@google.com
CPP:Update README on ICU version. Patch contributed by roubert@google.com

jarroda/libphonenumber-dotnet,weitzhandler/libphonenumber-csharp-1,caseykramer/libphonenumber-csharp,jarroda/libphonenumber-dotnet,fengzhyuan/libphonenumber-csharp,caseykramer/libphonenumber-csharp,fengzhyuan/libphonenumber-csharp,jagui/libphonenumber-csharp,weitzhandler/libphonenumber-csharp-1,caseykramer/libphonenumber-csharp,jarroda/libphonenumber-dotnet,fengzhyuan/libphonenumber-csharp,jagui/libphonenumber-csharp,fengzhyuan/libphonenumber-csharp,fengzhyuan/libphonenumber-csharp,jarroda/libphonenumber-dotnet,caseykramer/libphonenumber-csharp,caseykramer/libphonenumber-csharp,fengzhyuan/libphonenumber-csharp,jarroda/libphonenumber-dotnet,weitzhandler/libphonenumber-csharp-1,caseykramer/libphonenumber-csharp,weitzhandler/libphonenumber-csharp-1,weitzhandler/libphonenumber-csharp-1,jarroda/libphonenumber-dotnet,jagui/libphonenumber-csharp,caseykramer/libphonenumber-csharp,weitzhandler/libphonenumber-csharp-1,jarroda/libphonenumber-dotnet,weitzhandler/libphonenumber-csharp-1,jarroda/libphonenumber-dotnet,jagui/libphonenumber-csharp,fengzhyuan/libphonenumber-csharp