message: memo: added badge from isitmaintained.com
diff: [![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/nlohmann/json/master/LICENSE.MIT)
  [![Github Releases](https://img.shields.io/github/release/nlohmann/json.svg)](https://github.com/nlohmann/json/releases)
  [![Github Issues](https://img.shields.io/github/issues/nlohmann/json.svg)](http://github.com/nlohmann/json/issues)
  +[![Average time to resolve an issue](http://isitmaintained.com/badge/resolution/nlohmann/json.svg)](http://isitmaintained.com/project/nlohmann/json "Average time to resolve an issue")
  [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/289/badge)](https://bestpractices.coreinfrastructure.org/projects/289)
  - [Design goals](#design-goals)
  