Add documentation for adding a new libfuzzer fuzzer.

Author: bzbarsky-apple

docs/testing/fuzz_testing.md

@@ -0,0 +1,108 @@
+# Fuzz testing (Fuzzing)
+
+### Simple Fuzz test Setup
+
+The following example demonstrates how to use libfuzzer to write a simple
+fuzz test. Each fuzzer function is defined using `LLVMFuzzerTestOneInput(const uint8_t * data, size_t len)`. 
+
+The Fuzzer must be located in a Test Folder : `src/some_directory/tests/`
+
+```
+#include <cstddef>
+#include <cstdint>
+
+/**
+ *    @file
+ *      This file describes a Fuzzer for ...
+ */
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t * data, size_t len)
+{
+
+    // Instantiate values as needed
+    // Call target function for the fuzzer with the fuzzing input (data and len)
+
+    return 0;
+}
+
+```
+
+See
+[FuzzBase38Decode.cpp](https://github.com/project-chip/connectedhomeip/blob/master/src/setup_payload/tests/FuzzBase38Decode.cpp)
+for an example of a simple fuzz test.
+
+
+## Compiling and running
+
+-   Add to `src/some_directory/tests/BUILD.gn`
+
+    -   Example
+
+        ```
+
+        import("${chip_root}/build/chip/fuzz_test.gni")
+
+
+        if (enable_fuzz_test_targets) {
+            chip_fuzz_target("FuzzTargetName1") {
+                sources = [ "Fuzzer1.cpp" ]
+                public_deps = [
+                    // Dependencies go here.
+                ]
+            }
+            chip_fuzz_target("FuzzTargetName2") {
+                sources = [ "Fuzzer2.cpp" ]
+                public_deps = [
+                    // Dependencies go here.
+                ]
+            }
+        }
+        ```
+
+        - CHIP_FUZZ_TARGET : the name of the fuzz target
+        - SOURCES : file in the test folder containing the fuzzer implementation
+        - PUBLIC_DEPS : Code Dependencies needed to build fuzzer
+
+    -   Another example:
+        [src/setup_payload/tests/BUILD.gn](https://github.com/project-chip/connectedhomeip/blob/b367512f519e5e109346e81a0d84fd85cd9192f7/src/setup_payload/tests/BUILD.gn#L43)
+
+- Add to `src/BUILD.gn`
+    - Add the Fuzzing Target in this part of the code :
+        [src/BUILD.gn](https://github.com/project-chip/connectedhomeip/blob/b367512f519e5e109346e81a0d84fd85cd9192f7/BUILD.gn#L52)
+
+    -   Add Fuzzing Target like that
+
+        ```
+        if (enable_fuzz_test_targets) {
+            group("fuzz_tests") {
+            deps = [
+                "${chip_root}/src/credentials/tests:fuzz-chip-cert",
+                "${chip_root}/src/lib/core/tests:fuzz-tlv-reader",
+                "${chip_root}/src/lib/dnssd/minimal_mdns/tests:fuzz-minmdns-packet-parsing",
+                "${chip_root}/src/lib/format/tests:fuzz-payload-decoder",
+                "${chip_root}/src/setup_payload/tests:fuzz-setup-payload-base38",
+                "${chip_root}/src/setup_payload/tests:fuzz-setup-payload-base38-decode",
+                // ADD HERE YOUR FUZZING TARGET 
+                "${chip_root}/some_directory/tests:FuzzTargetName"
+                ]
+            }
+        }
+        ```
+
+-   Build all fuzzers
+    ```
+    ./scripts/build/build_examples.py --target <host>-<compiler>-tests-asan-libfuzzer-clang build
+    ```
+    e.g.
+    ```
+    ./scripts/build/build_examples.py --target darwin-arm64-tests-asan-libfuzzer-clang build
+    ```
+    ** Make sure to put the right host and compiler 
+-   Fuzzers binaries are compiled into:
+    -   `out/<host>-<compiler>-tests-asan-libfuzzer-clang/tests`
+    -   e.g. `darwin-arm64-tests-asan-libfuzzer-clang`
+
+- Running the fuzzer with a corpus 
+    - ```path_to_fuzzer_in_test_folder path_to_corpus ```
+
+