[dev-context] CGModules update: Letter module version 2020.01.20 published.

Wolfgang Schuster submitted an update to the Letter module package. Version number: 2020.01.20 License type: GNU General Public License Wants to be in ConTeXt distribution: no Wants to be synced with CTAN: no Summary description: Writing letters with ConTeXt. Announcement text: ---------------------------------------------------------------------- README was missing in the previous release. ----------------------------------------------------------------------