Added README.md for Github.

README.md

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+The Scyther tool repository
+===========================
+
+This README describes the organization of the repository of the Scyther
+tool for security protocol analysis. Its intended audience are
+interested users and future developers of the Scyther tool, as well as
+protocol modelers. For installation and usage instructions of the
+Scyther tool see:
+<http://people.inf.ethz.ch/cremersc/scyther/index.html>.
+
+Developing
+----------
+
+We use Linux during the development of Scyther, but development on
+Windows and MAC OS X should be equally feasible. Note that the below
+instructions are written from a Linux/Ubuntu perspective, and probably
+need modifications for other platforms.
+
+Scyther is written partly in Python (for the GUI, using wxPython) and
+partly in C (for the backend). 
+
+In order to run the tool from a repository checkout, it is required to
+compile the C sources into a working binary for the backend.  The
+simplest way to achieve this is to run the `build.sh` script in the
+`./src` directory. This script compiles a binary version of the tool on
+the native platform. Thus, in the Linux case, it should produce
+`./src/scyther-linux`. This file is automatically copied to the related
+directory under `./gui`, and if successful you can attempt to run
+`./gui/scyther-gui.py` to use the graphical user interface.
+
+The build process depends on the following
+(Debian/Ubuntu) packages:
+
+  * `cmake`
+  * `build-essential`
+  * `flex`
+  * `bison`
+
+If you are using Ubuntu, installing these may be as simple as running
+
+`sudo apt-get install cmake build-essential flex bison`
+
+In case you also want to be able to compile Windows binaries from Linux,
+you also need:
+
+  * `mingw32`
+
+Note that welcome all contributions, e.g., further protocol models. Just send
+us a pull request.
+
+
+Manual
+------
+
+We are currently rewriting the manual. Bear with us for a second and
+have a look at the example protocol models.
+
+
+Protocol Models
+---------------
+
+The protocol models have the extension `.spdl` and can be found in the following directories:
+
+  * `./gui/Protocols`, containing the officially released models, and
+  * `./protocols`, containing models currently under development.
+
+License
+-------
+
+Currently these Scyther sources are licensed under the GPL 2, as indicated in
+the source code. Contact Cas Cremers if you have any questions.
+