docs: describe how to reproduce errors reported by oss-fuzz

Add a description for current oss-fuzz setup and write down the
commands needed to reproduce fuzzer reported errors:
- "Official way" in case exact oss-fuzz environment is necessary.
- "Simple way" for local tinkering.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

fuzz/readme.md

@@ -0,0 +1,58 @@
+# About fuzzing
+
+Fuzzing is done by [OSS-Fuzz](https://google.github.io/oss-fuzz/).
+It works by creating a project-specific binary that combines fuzzer
+itself and a project provided entry point named
+`LLVMFuzzerTestOneInput()`. When invoked, this executable either
+searches for new test cases or runs an existing test case.
+
+File `fuzz/bpf-object-fuzzer.c` defines an entry point for the robot:
+- robot supplies bytes supposed to be an ELF file;
+- wrapper invokes `bpf_object__open_mem()` to process these bytes.
+
+File `scripts/build-fuzzers.sh` provides a recipe for fuzzer
+infrastructure on how to build the executable described above (see
+[here](https://github.com/google/oss-fuzz/tree/master/projects/libbpf)).
+
+# Reproducing fuzzing errors
+
+## Official way
+
+OSS-Fuzz project describes error reproduction steps in the official
+[documentation](https://google.github.io/oss-fuzz/advanced-topics/reproducing/).
+Suppose you received an email linking to the fuzzer generated test
+case, or got one as an artifact of the `CIFuzz` job (e.g. like
+[here](https://github.com/libbpf/libbpf/actions/runs/16375110681)).
+Actions to reproduce the error locally are:
+
+```sh
+git clone --depth=1 https://github.com/google/oss-fuzz.git
+cd oss-fuzz
+python infra/helper.py pull_images
+python infra/helper.py build_image libbpf
+python infra/helper.py build_fuzzers --sanitizer address libbpf <path-to-libbpf-checkout>
+python infra/helper.py reproduce libbpf bpf-object-fuzzer <path-to-test-case>
+```
+
+`<path-to-test-case>` is usually a `crash-<many-hex-digits>` file w/o
+extension, CI job wraps it into zip archive and attaches as an artifact.
+
+To recompile after some fixes, repeat the `build_fuzzers` and
+`reproduce` steps after modifying source code in
+`<path-to-libbpf-checkout>`.
+
+Note: the `build_fuzzers` step creates a binary
+`build/out/libbpf/bpf-object-fuzzer`, it can be executed directly if
+your environment is compatible.
+
+## Simple way
+
+From the project root:
+
+```sh
+SKIP_LIBELF_REBUILD=1 scripts/build-fuzzers.sh
+out/bpf-object-fuzzer <path-to-test-case>
+```
+
+`out/bpf-object-fuzzer` is the fuzzer executable described earlier,
+can be run with gdb etc.