Add documentation how to use fuzzers

[nlohmann]

From #3475 (comment):

• Add documentation how to use AFL.
• Add documentation how to use LibFuzzer.
• Add documentation how OSS-Fuzz is relying on this repo.

[falbrechtskirchinger]

find tests/data -size -5k -name *.bjdata | xargs -I{} cp "{}" fuzz-testing/testcases
find: ‘tests/data’: No such file or directory

Remember to document the test data location.

[falbrechtskirchinger]

Also:

Note: AFL hasn't been updated for a couple of years; while it should still work fine, a more complex fork with a variety of improvements and additional features, known as AFL++, is available from other members of the community and might be worth checking out.

https://github.com/AFLplusplus/AFLplusplus

[nlohmann]

I think that was the reason I could not get it running quickly on macOS. So LibFuzzer it is. I'll check it out later today.

[nlohmann]

I added a PR: #3478

[nlohmann]

Done.