Using .md extension/Markdown syntax for modules README

Renaming all README files for all top level modules to README.md,
applying no content change at this stage to be able to track history
of the files. These files should be edited to use correct Markdown
syntax later.

Change-Id: I542fa3f3d32072156f16eaad2211a397cc212665
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/112977
Tested-by: Jenkins
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>