DOCUMENTATION: add markdown links in the readme files
It would be very nice to have the link to the ADS entry when a paper is cited.
This is done like that:
This was done accordingly to Poppenhaeger et. al. (2020) [^Poppenhaeger-et-al-20], and we improved it while mixing with Tu et al. (2015) [^Tu-et-al-20]. You can also find ... [^Poppenhaeger-et-al-20]: [Poppenhaeger et. al. 2020](https://arxiv.org/abs/2005.10240) [^Tu-et-al-20]: [Tu et. al. 2020](https://arxiv.org/abs/2005.10240)