README enhancement

In my humble opinion, the README file provided with Orekit source code is totally old-fashioned:

It uses the plain text format rather than a lightweight markup language such as Markdown or reStructuredText.
It is poor in information (no indication is provided, for example, on how to contribute, or how to report a bug).

Several initiatives have attempted to list good practices regarding the content of a README file and/or to provide README templates:

I think that Orekit merits such modern README file.

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