Migrate the 'overview' document

Author: ningli

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+## Overview
+
+### Introduction
+
+The 2DECOMP&FFT library is a software framework in Fortran to build large-scale parallel applications. It is designed for applications using three-dimensional structured mesh and spatially implicit numerical algorithms. At the foundation it implements a general-purpose 2D pencil decomposition for data distribution on distributed-memory platforms. On top, it provides a highly scalable and efficient interface to perform three-dimensional distributed FFTs. The library is optimised for supercomputers and scales well to hundreds of thousands of cores. It relies on MPI but provides a user-friendly programming interface that hides communication details from application developers.
+
+### Features
+
+Here is a list of 2DECOMP&FFT's main features:
+
+* General-purpose 2D pencil decomposition module to support building large-scale parallel applications on distributed memory systems.
+* Highly scalable and efficient distributed Fast Fourier Transform module, supporting three dimensional FFTs (both complex-to-complex and real-to-complex/complex-to-real).
+* Halo-cell support allowing explicit message passing between neighbouring blocks.
+* Parallel I/O module to support the handling of large data sets.
+* Shared-memory optimisation on the communication code for multi-core systems.
+
+2DECOMP&FFT distinguishes itself from many other popular distributed FFT libraries by exposing its communication APIs upon which many other parallel algorithms can be built.
+
+2DECOMP&FFT is designed to be:
+
+* **Scalable** - The library and applications built upon it are known to scale to o(10^5) cores on major supercomputers.
+* **Flexible** - Software framework to support building higher-level libraries and many types of applications.
+* **User-friendly** - Black-box implementation and very clean application programming interface hiding most communication details from applications.
+* **Portable** - Code tested on many major supercomputing architectures. The FFT library interfaces with almost every popular external FFT implementations.
+
+### History
+
+This software package was originally derived from several projects funded under the HECToR Distributed Computational Science and Engineering (dCSE) programme operated by NAG Ltd. HECToR - a UK Research Councils' high end computing service - served as the UK's national supercomputer for open science between 2008 and 2014.
+
+The active development of this library completed in 2012. It has been in production use in many research applications since then. The code appears to be in a very good quality with almost no major bugs reported.
+
+Since August 2021, this project is hosted in NAG's official GitHub account to facilitate future development and maintenance.
+
+### Citation
+
+If you wish to cite this work, you are recommended to use the following paper:
+
+* N. Li and S. Laizet, "2DECOMP&FFT – A highly scalable 2D decomposition library and FFT interface", Cray User Group 2010 conference, Edinburgh, 2010.