README | add Zenodo badge

Author: flokno

README.md

@@ -5,6 +5,7 @@ Temperature Dependent Effective Potentials (TDEP)
 ![GitHub Release](https://img.shields.io/github/v/release/tdep-developers/tdep)
 ![GitHub License](https://img.shields.io/github/license/tdep-developers/tdep)
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 Briefly summarized, the package provides all the tools you need to build accurate model Hamiltonians for finite temperature lattice dynamics from first principles. TDEP includes several programs for different tasks: