Improve README

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README.md

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-# opm-common
-Contains common components used throughout all of OPM,
-in particular CMake modules for the build system.
+# opm-common: Tools for Eclipse reservoir simulation files
+
+The Open Porous Media (OPM) software suite provides libraries and
+tools for modeling and simulation of porous media processes,
+especially  for simulating CO2 sequestration and improved and enhanced
+oil recovery. The Eclipse file format is widely used in the reservoir
+simulation  community. This package provides a library containing code
+for processing files in Eclipse format, Python binding for accessing
+Eclipse files and  utility code used by other OPM modules (e.g. CMake
+modules used).
+
+License
+-------
+
+The library is distributed under the GNU General Public License,
+version 3 or later (GPLv3+).
+
+Requirements
+-----------
+
+- C++17 compatible compiler
+- CMake >= 3.10