Update README.md with warning of pending header changes

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 Vulkan header files and API registry
 
+## Advance Notice of Pending Header Change (posted 2020-04-06)
+
+Shortly after 2020-04-24, we will be removing the automatically generated
+`VK_*_BEGIN_RANGE`, `VK_*_END_RANGE`, and `VK_*_RANGE_SIZE` tokens
+from the Vulkan headers.
+These tokens are currently defined for some enumerated types, but are
+explicitly not part of the Vulkan API.
+They existed only to support some Vulkan implementation internals, which no
+longer require them.
+We will be accepting comments on this topic in
+https://github.com/KhronosGroup/Vulkan-Docs/issues/1230, but we strongly
+suggest any external projects using these tokens immediately migrate away
+from them.
+
 ## Repository Content
 
 The contents of this repository are largely obtained from other repositories and are