Wording for MIT license and external contributors

Author: LefterisJP

LICENSE GPLV3_LICENSE

@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2015 <copyright holders>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

LICENCE

@@ -43,10 +43,20 @@ See [TODO](https://github.com/ethereum/cpp-ethereum/wiki/TODO)
 
 ### License
 
-See [LICENSE](LICENSE)
+All new contributions are under the [MIT license](http://opensource.org/licenses/MIT).
+See [LICENSE](LICENSE). Some old contributions are under the [GPLv3 license](http://www.gnu.org/licenses/gpl-3.0.en.html). See [GPLV3_LICENSE](GPLV3_LICENSE).
 
 ### Contributing
 
+All new contributions are added under the MIT License. Please refer to the `LICENCSE` file in the root directory.
+To state that you accept this fact for all of your PRs please add yourself to the list of external contributors
+like in the example below.
+
+#### External Contributors
+- *Name Surname* (**email@domain**)
+
+#### Contribution guideline
+
 All development goes in develop branch - please don't submit pull requests to master.
 
 Please read [CodingStandards.txt](CodingStandards.txt) thoroughly before making alterations to the code base. Please do *NOT* use an editor that automatically reformats whitespace away from astylerc or the formatting guidelines as described in [CodingStandards.txt](CodingStandards.txt).