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

[gregmarr]

Add to CONTRIBUTING.md that make pretty is required for test updates, as not doing so will result in a failure in the amalgamation check.

Pull request checklist

• Changes are described in the pull request, or an existing issue is referenced.
• [n/a] The test suite compiles and runs without error.
• [n/a] Code coverage is 100%. Test cases can be added by editing the test suite.
• [n/a] The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

[coveralls]

coverage: 100.0%. remained the same when pulling 3128fae on gregmarr:patch-1 into a0c1318 on nlohmann:develop.

[nlohmann]

Oof, Clang-Tidy again has some complaints... I'll check before merging this.

[gregmarr]

The clang-tidy complaints are in files not touched by this PR.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks a lot!