Update README.md

[dalibormesaric]

Updated size of string from 10KB to 20KB according to

Note: Strings are stored as UTF-16 characters in .NET so each char takes two bytes in memory.

but also after running

Console.WriteLine(
    System.Text.Encoding.Unicode.GetByteCount(
        new String('x', 10 * 1024)));

// Answer: 20480

[dalibormesaric]

Also created dotnet/AspNetCore.Docs#33937