Update README.md

Author: ArnauPrat

README.md

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 The LDBC-SNB Data Generator (DATAGEN) is the responsible of providing the data sets used by all the LDBC benchmarks. This data generator is designed to produce directed labeled graphs that mimic the characteristics of those graphs of real data. A detailed description of the schema produced by datagen, as well as the format of the output files, can be found in the latest version of official [LDBC SNB specification document](https://github.com/ldbc/ldbc_snb_docs).