add a README.md which is used by GitHub.

README.md

+The ROOT System
+===============
+
+The ROOT system provides a set of OO frameworks with all the functionality
+needed to handle and analyze large amounts of data in a very efficient way.
+Having the data defined as a set of objects, specialized storage methods are
+used to get direct access to the separate attributes of the selected objects,
+without having to touch the bulk of the data. Included are histograming
+methods in an arbitrary number of dimensions, curve fitting, function
+evaluation, minimization, graphics and visualization classes to allow
+the easy setup of an analysis system that can query and process the data
+interactively or in batch mode, as well as a general parallel processing
+framework, PROOF, that can considerably speed up an analysis.
+
+Thanks to the built-in CINT C++ interpreter the command language,
+the scripting, or macro, language and the programming language are all
+C++. The interpreter allows for fast prototyping of the macros since it
+removes the, time consuming, compile/link cycle. It also provides a good
+environment to learn C++. If more performance is needed the interactively
+developed macros can be compiled using a C++ compiler via a machine
+independent transparent compiler interface called ACliC.
+
+The system has been designed in such a way that it can query its databases
+in parallel on clusters of workstations or many-core machines. ROOT is
+an open system that can be dynamically extended by linking external
+libraries. This makes ROOT a premier platform on which to build data
+acquisition, simulation and data analysis systems.
+
+ROOT is available under the LGPL license.