README: add code from BoringSSL paragraph

ec/native/README.md

@@ -1,7 +1,7 @@
 # Generated code from fiat
 
 This directory includes several files ("p*.h") that are generated by
-[fiat](https://github.com/mit-plv/fiat-crypto). The Makefile provides
+[fiat](https://github.com/mit-plv/fiat-crypto). The GNUmakefile provides
 targets to generate these files.
 
 The file "inversion_template.h" is copied from the fiat-crypto repository
@@ -10,3 +10,9 @@ and has some modifications: the "inversion" function is declared "static",
 and the convenience function "inversion" is provided.
 
 The "*_stubs.c" files are handcrafted.
+
+# Code from BoringSSL
+
+The code in "point_operations.h", "curve25519_tables.h", and large parts of
+"curve25519_stubs.c" originate from BoringSSL. Minor adjustments have been done
+manually.