feat: added GDB (closes Add gdb #13)

README.md

@@ -26,6 +26,7 @@ Contributions are very welcome - please follow the [guidelines](CONTRIBUTING.md)
 - [COCONUT](https://coconut.naturalproducts.net/): is an open source project for Natural Products (NPs) storage, search and analysis.
 - [Crystallography Open Database](http://www.crystallography.net/cod/): open-access collection of crystal structures of organic, inorganic, metal-organic compounds and minerals, excluding biopolymers. [They also derived SMILES for some compounds.](https://doi.org/10.1186/s13321-018-0279-6)
 - [Enamine HTS collection](https://enamine.net/compound-collections/screening-collection/hts-collection):  1 930 980 diverse screening compounds (37 billion molecules in 2D and 4.5 billion in 3D)
+- [GDB](https://gdb.unibe.ch/downloads/): enumeration of molecules according to simple (feasibility and stability) rules
 - [GNPS](https://gnps.ucsd.edu/ProteoSAFe/static/gnps-splash.jsp): mass spectrometry database with focus on natural products, contains untargeted (unlabelled) data.
 - [MoNA](https://mona.fiehnlab.ucdavis.edu/): mass spectrometry database of real and predicted spectra for known compounds.
 - [nCov-Group Data Repository](https://2019-ncovgroup.github.io/data/#dataset-downloads): SMILES, fingerprints, descriptors, and images of millions of compounds.