updated librecad.dox

I updated librecad.dox with doxygen version 1.8.4. Added README.md as a main page of documentation.

Changes to new librecad.dox :
1) OUTPUT_DIRECTORY   =  Doc
2) USE_MDFILE_AS_MAINPAGE  =  README.md
3) INPUT = librecad  README.md
4) GENERATE_TREEVIEW =  YES

Here is a pull request   https://github.com/LibreCAD/LibreCAD/pull/398

Thanks for merging the request. :)

thanks for requesting