License headers on top of each source file (and README)

I have followed those recommendations : https://www.gnu.org/licenses/agpl-3.0.de.html

@librelois @c-geek @Insoleet

keys/ed25519.rs

+//  Copyright (C) 2017  The Duniter Project Developers.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as
+// published by the Free Software Foundation, either version 3 of the
+// License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
 //! Provide wrappers around ed25519 keys and signatures
 //!
 //! Key pairs can be generated with [`KeyPairGenerator`].