add example for Extract convertDatafunc

[xavave]

excellent project
just a new example in case someone is looking for it:
rmWorkSheet.Extract<RawMaterial>() // Here we can chain multiple definition for the columns .WithProperty(p => p.Id, "A") .WithProperty(p => p.Name, "B", fnTrimString) .WithProperty(p => p.Code, "C", fnTrimString)

fnTrimString is converting, fot this example, input object to string
with
Func<object, string> fnTrimString = objStr => { if (objStr == null) return ""; return objStr.ToString().Trim(); };