Mismatch between README.md and AddTimestamp.cs

[ied206]

This PR solves mismatch of format string between README.md and AddTimestamp.cs

README.md states that timestamp is saved with format string of yyyy-MM-dd.

### Adds the following attribute to your assembly

    [assembly: Timestamp("yyyy-MM-dd")]

So for example if you compile your assembly on the 20th of November 2013 the timestamp will be.

    [assembly: Timestamp("2013-11-20")]

AddTimestamp.cs is using format string of yyyy-MM-ddTHH:mm:ss.fffZ.

        void CreateTempAssemblyInfo()
        {
            var timestampUsage = $@"
// <auto-generated/>
using Timestamp;
[assembly: Timestamp(""{DateTime.UtcNow:yyyy-MM-ddTHH:mm:ss.fffZ}"")]
";

Testing reveals nuget package's behavior is consistent with AddTimestamp.cs, so update README.md.

[SimonCropp]

@ied206 thanks