The RTM build of the SDK for the Windows 10 Creators Update is now available. It's officially known as an Insider Preview, but since 15063 is the RTM build for the Update, this is clearly the version of the software development kit (SDK) that devs are going to get.
Microsoft also announced today that the Creators Update will begin rolling out to PCs on April 11. The company has said previously that PC users would get it first, and that other platforms would follow. Clearly, it was referring to phones, as Xbox One users actually got the update today.
The new build replaces Preview SDK build 15052, which was made available 15 days ago. It comes alongside a new Windows 10 Mobile emulator of the same build number, and ISO images of the PC build were made available too.
In fact, this SDK has been available to download for over a week, if you had the download link. The change here is that it's now listed on the Windows Insider Program page.
There also hasn't been an official announcement about this at all, as there is typically a post on the Windows Blog. If you head over to the download page and click the link for the release notes, it will take you to the 15052 announcement.