Documentation: fpga: dfl: add link address of feature id table

This patch adds the link address of feature id table in documentation.
Signed-off-by: Tianfei zhang <tianfei.zhang@intel.com>
Reviewed-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>
Acked-by: Moritz Fischer <mdf@kernel.org>
Acked-by: Wu Hao <hao.wu@intel.com>
Link: https://lore.kernel.org/r/20220419032942.427429-3-tianfei.zhang@intel.comSigned-off-by: Xu Yilun <yilun.xu@intel.com>

Documentation/fpga/dfl.rst

@@ -502,6 +502,11 @@ Developer only needs to provide a sub feature driver with matched feature id.
 FME Partial Reconfiguration Sub Feature driver (see drivers/fpga/dfl-fme-pr.c)
 could be a reference.
 
+Please refer to below link to existing feature id table and guide for new feature
+ids application.
+https://github.com/OPAE/dfl-feature-id
+
+
 Location of DFLs on a PCI Device
 ================================
 The original method for finding a DFL on a PCI device assumed the start of the