Add note about submodule in README

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

README.md

@@ -10,3 +10,13 @@ other / multiple targets, check their respective README / build files
 
 Applicable license is individual to each IP core / project and is mentionnned
 in the IP core / example directory itself and in each file.
+
+
+Note about submodules
+---------------------
+
+Make sure to clone this repository with all its submodules by using
+the `-recursive` option when cloning or running `git submodule init` and
+`git submodule update` after checkout.
+
+Also make sure to update submodules when pulling from upstream.