Add DFTB+ to package index

[awvwgk]

Disclaimer: this is a project I'm personally involved with.

This PR adds an entry for DFTB+

• GH organisation of the project: https://github.com/dftbplus
• Project homepage: https://dftbplus.org/
• the projects builds with CMake
• supports GCC7.5, GCC8.4, GCC9.2, GCC10.1, Intel 18, Intel 19, NAG 7
• MPI and/or OpenMP parallel
• documentation for DFTB+
  • developer docs: https://dftbplus-develguide.readthedocs.io/en/latest/
  • user docs: https://dftbplus-recipes.readthedocs.io/en/latest/

See #68

[certik]

Great addition, thanks!