README: Describe what LibreOffice is about

Before saying how to develope it, mention what it's all about.
The text was shamelessly stolen from OpenHUB LibreOffice project
site: [1].

* [1] https://www.openhub.net/p/libreoffice

Change-Id: I9e018e441c4d3f176d1d21eddb7a65429a9f9fd2
Reviewed-on: https://gerrit.libreoffice.org/20252Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: jan iversen <jani@documentfoundation.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

README.md

 # LibreOffice
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+LibreOffice is an integrated office suite based on copyleft licenses
+and compatible with most document formats and standards. Libreoffice
+is backed by The Document Foundation, which represents a large
+independent community of enterprises, developers and other volunteers
+moved by the common goal of bringing to the market the best software
+for personal productivity. LibreOffice is open source, and free to
+download, use and distribute.
+
 A quick overview of the LibreOffice code structure.
 
 ## Overview