target/sdk: update README.SDK to explain dependency handling

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/sdk/files/README.SDK

@@ -4,4 +4,8 @@ having to compile your own toolchain or any of the libraries
 included with LEDE.
 
 To use it, just put your buildroot-compatible package directory
-in the subdir 'package/' and run 'make' from this directory.
+(including its dependencies) in the subdir 'package/' and run
+'make' from this directory.
+
+To make dependency handling easier, you can use ./scripts/feeds
+to install any core package that you need