2020-04: Exchange ready for external security audit

We received a grant from NLnet foundation to pay for an external security audit of the GNU Taler exchange cryptography, code and documentation. We spent the last four months preparing the code, closing almost all of the known issues, performing static analysis, fixing compiler warnings, improving test code coverage, fuzzing, benchmarking, and reading the code line-by-line. Now, we are now ready to start the external audit. This April, CodeBlau will review the code in the Master branch tagged CodeBlau-NGI-2019 and we will of course make their report available in full once it is complete. Thanks to NLnet and the European Commission's Horizion 2020 NGI initiative for funding this work.