Update README reflecting process of preview extensions removal (#3094)

Co-authored-by: Ashbaugh, Ben <ben.ashbaugh@intel.com>
Co-authored-by: Sven van Haastregt <sven.vanhaastregt@arm.com>

Signed-off-by: Sidorov, Dmitry <dmitry.sidorov@intel.com>

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@739b46c4d90015a

Author: MrSidims

llvm-spirv/README.md

@@ -265,3 +265,18 @@ creating a pull request or raising an issue on GitHub.
 As mentioned earlier there are branches `llvm_release_*` that get backported
 changes. Those changes if exists are released automatically by github CI on
 monthly basis in a format `<llvm_major>.<llvm_minor>.<latest patch +1>`.
+
+## Deprecation of "preview extensions"
+
+In a case if a "preview extension" has to be deprecated, as the first step one
+should disable support for forward translation of the extension in the main branch
+(this will prevent new SPIR-V modules from being generated using the extension).
+Meanwhile support reverse translation for the extension should be continued
+(this retains compatibility with existing SPIR-V modules).
+  * Addition of deprecation warning for the extension is not required.
+  * We encourage backporting the changes to other branches to speed up removal, but this is not required.
+
+After at least one release cycle one may remove support for reverse translation in the main branch as well,
+at which point support for the "preview extension" is considered removed.
+
+These are guidelines, not requirements, and we will consider exceptions on a case-by-case basis.