Add to CONTRIBUTING.md that make pretty is required for test update…

…s. (#4045)

.github/CONTRIBUTING.md

@@ -32,7 +32,7 @@ To make changes, you need to edit the following files:
 
 1. [`include/nlohmann/*`](https://github.com/nlohmann/json/tree/develop/include/nlohmann) - These files are the sources of the library. Before testing or creating a pull request, execute `make amalgamate` to regenerate `single_include/nlohmann/json.hpp` and `single_include/nlohmann/json_fwd.hpp`.
 
-2. [`tests/src/unit-*.cpp`](https://github.com/nlohmann/json/tree/develop/tests/src) - These files contain the [doctest](https://github.com/onqtam/doctest) unit tests which currently cover [100 %](https://coveralls.io/github/nlohmann/json) of the library's code.
+2. [`tests/src/unit-*.cpp`](https://github.com/nlohmann/json/tree/develop/tests/src) - These files contain the [doctest](https://github.com/onqtam/doctest) unit tests which currently cover [100 %](https://coveralls.io/github/nlohmann/json) of the library's code.  Before creating a pull request, execute `make pretty` to make sure that the style is correct, as this will be checked by the CI. 
 
    If you add or change a feature, please also add a unit test to this file. The unit tests can be compiled and executed with