Windows Phone 8.1 is still in the early stages of rolling out to devices in over-the-air updates, but Microsoft is hard at work at future updates, including general distribution releases also known as GDRs. Those GDR updates, usually called just 'Update' or 'Update 2', bring patches, minor fixes, a few new features and enable additional hardware options for manufacturers (OEMs).
Evidently, references to GDR1 are found on the Windows Phone dev center under the OEM documentation. Unfortunately, if you try to click the links you are asked to log in, making this restricted to just OEM partners.
The good news here though is GDR1 is completed enough that the documentation for manufacturing partners is now available. The prospects of an official Update for Windows Phone 8.1 coming in the near future are certainly high and hopefully Microsoft has more to reveal on this update soon.
Windows Phone 8.1 Update so far is a bit of a mystery in terms of features for consumers, although native Folder support on the Start screen is confirmed as one of those additions. GDRs though are typically light on features and focus more on underlying fixes and improvements. We reported previously that Microsoft is planning on two OS updates this year for Windows Phone 8.1.
Finally, Microsoft is expected to release GDR1 through their Preview for Developers program as they have in the past e.g. Windows Phone 8.0 Update 3.