fuzz: add a quick README to try and document how to test new fuzzers
Differential Revision: https://phab.mercurial-scm.org/D2633
How to add fuzzers (partially cribbed from oss-fuzz[0]):
1) git clone https://github.com/google/oss-fuzz
2) cd oss-fuzz
3) python infra/helper.py build_image mercurial
4) docker run --cap-add=SYS_PTRACE -it -v $HG_REPO_PATH:/hg-new \
gcr.io/oss-fuzz/mercurial bash
5) cd /src
6) rm -r mercurial
7) ln -s /hg-new mercurial
8) cd mercurial
9) compile
0: https://github.com/google/oss-fuzz/blob/master/docs/new_project_guide.md