Document .gitignore split

This is taken verbatim from the linux kernel .gitignore

Signed-off-by: Eduardo Lima (Etrunko) <eduardo.lima@intel.com>

.gitignore

@@ -1,3 +1,12 @@
+#
+# NOTE! Don't add files that are generated in specific
+# subdirectories here. Add them in the ".gitignore" file
+# in that subdirectory instead.
+#
+# NOTE! Please use 'git ls-files -i --exclude-standard'
+# command after changing this file, to see if there are
+# any tracked files which get ignored after the change.
+#
 *.o
 *.a
 *.la