Preview at WWDC likely in June, followed by September launch.
Apple Seeds Second OS X 10.11.6 El Capitan Beta to Developers
The OS X 10.11.6 update, build 15G12a, can be downloaded through the Apple Developer Center or the Software Update mechanism in the Mac App Store.
OS X 10.11.6, like OS X 10.11.5 and the El Capitan updates that came before it, appears to focus mainly on performance improvements and bug fixes. According to Apple's release notes for the first beta, OS X 10.11.6 improves the stability, compatibility, and security of Macs.
No outward-facing bug fixes or changes were found in the first beta but we'll update this post if any new features are discovered in the second beta.