README.rst: Include some notes about syncing uapi headers

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

README.rst

@@ -13,6 +13,24 @@ but a new libdrm will always work with an older kernel.
 libdrm is a low-level library, typically used by graphics drivers such as
 the Mesa drivers, the X drivers, libva and similar projects.
 
+Syncing with the Linux kernel headers
+-------------------------------------
+
+The library should be regularly updated to match the recent changes in the
+`include/uapi/drm/`.
+
+libdrm maintains a human-readable version for the token format modifier, with
+the simpler ones being extracted automatically from `drm_fourcc.h` header file
+with the help of a python script.  This might not always possible, as some of
+the vendors require decoding/extracting them programmatically.  For that
+reason one can enhance the current vendor functions to include/provide the
+newly added token formats, or, in case there's no such decoding
+function, to add one that performs the tasks of extracting them.
+
+For simpler format modifier tokens there's a script (gen_table_fourcc.py) that
+creates a static table, by going over `drm_fourcc.h` header file. The script
+could be further modified if it can't handle new (simpler) token format
+modifiers instead of the generated static table.
 
 Compiling
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