Improve build instructions for fuzzer (#3364)

README.md

@@ -324,8 +324,7 @@ The script `utils/git-sync-deps` can be used to checkout and/or update the
 contents of the repos under `external/` instead of manually maintaining them.
 
 ### Build using CMake
-You can build The project using [CMake][cmake] to generate platform-specific
-build configurations.
+You can build the project using [CMake][cmake]:
 
 ```sh
 cd <spirv-dir>
@@ -333,8 +332,36 @@ mkdir build && cd build
 cmake [-G <platform-generator>] <spirv-dir>
 ```
 
-Once the build files have been generated, build using your preferred
-development environment.
+Once the build files have been generated, build using the appropriate build
+command (e.g. `ninja`, `make`, `msbuild`, etc.; this depends on the platform
+generator used above), or use your IDE, or use CMake to run the appropriate build
+command for you:
+
+```sh
+cmake --build . [--config Debug]  # runs `make` or `ninja` or `msbuild` etc.
+```
+
+#### Note about the fuzzer
+
+The SPIR-V fuzzer, `spirv-fuzz`, can only be built via CMake, and is disabled by
+default. To build it, clone protobuf and use the `SPIRV_BUILD_FUZZER` CMake
+option, like so:
+
+```sh
+# In <spirv-dir> (the SPIRV-Tools repo root):
+git clone https://github.com/protocolbuffers/protobuf external/protobuf
+pushd external/protobuf
+git checkout v3.7.1
+popd
+
+# In your build directory:
+cmake [-G <platform-generator>] <spirv-dir> -DSPIRV_BUILD_FUZZER=ON
+cmake --build . --config Debug
+```
+
+You can also add `-DSPIRV_ENABLE_LONG_FUZZER_TESTS=ON` to build additional
+fuzzer tests.
+
 
 ### Build using Bazel
 You can also use [Bazel](https://bazel.build/) to build the project.