Flex-Rigid Design Links

Here is an Altium article titled "Best Practice of Flex Circuit Design"

https://www.altium.com/rigid-flex-circuits-resources/best-practice-flex-circuit-design

It's a bit dated, it was published when AD16 was released, however it is still relevant and has some of the best graphics I have seen related to flex designs.

Also see: 

Rigid-Flex Design - Altium Online Documentation
Coverlay Polygon - Altium Online Documentation
Bending Line - Altium Online Documentation

More flex resources:

Flexible Circuit Technologies is a highly regarded Flex resource.
Wurth Elektronik's also has some good stuff to share about designing with Flex-Rigid.

Video:
Advanced Rigid Flex Circuit Constructions - epec
Minimum Bend Radius Calculations

https://www.leiton.de/leiton-tools-flex-bendingradius.html
Best Technology - Asia


Tips:

Define Rigid-Flex Regions using View > Board Planning Mode

Select View > Board Planning Mode, then Select > Design > Define Split Line 

Snapping Options (Try Track Lines and a little cheese)

To delete a splitting line wiggle the line then press the delete key.

Enable All Objects in the Selection Filter to grab Splitting lines.


Show all Single Layers and Enable Regions to Define Stackup Regions

That's it !