docs: Add short windows section to dev guide

The dev guide had no references for how to build or test for windows at
all. Add a short section with some minimal info and guidance for where
to find more detailed resources.

I didn't want to write extensive docs on this, especially as they're
likely to be unmaintained and go stale, so the user is expected to look
at the CI automation files to extract required library URLs, commands
etc.

Signed-off-by: Craig Ringer <[email protected]>

DEVELOPER_GUIDE.md

@@ -27,6 +27,7 @@ changes to Fluent Bit.
     - [Output](#output)
     - [Config Maps](#config-maps)
   - [Testing](#testing)
+    - [Building and Testing on Windows](#building-and-testing-on-windows)
     - [Valgrind](#valgrind)
   - [Need more help?](#need-more-help)
 
@@ -593,6 +594,8 @@ Note that Fluent Bit uses Cmake 3 and on some systems you may need to invoke it
 
 To set up and build your environment, please refer to the packaging containers for a dependency list: <https://github.com/fluent/fluent-bit/tree/master/packaging/distros>.
 
+See [Building and Testing on Windows](#building-and-testing-on-windows) for Windows instructions.
+
 A simple container-based script [`run_code_analysis.sh`](./run_code_analysis.sh) is provided to run unit tests using <https://github.com/marketplace/actions/cmake-swiss-army-knife>.
 
 ```shell
@@ -646,6 +649,88 @@ Test sds_printf...                              [   OK   ]
 SUCCESS: All unit tests have passed.
 ```
 
+### Building and testing on Windows
+
+Fluent Bit is built with MSVC and CMake on Windows.
+
+Windows builds of Fluent Bit override the set of enabled plugins. See
+[`cmake/windows-setup.cmake`](./cmake/windows-setup.cmake) for the override
+list and other Windows-specific build rules.
+
+#### Using a Github Action
+
+For developers without convenient access to a Windows build machine the easiest
+way to build a branch of Fluent Bit on Windows is to fork the `fluent-bit`
+repository and enable workflow runs. The
+[`.github/workflows/pr-windows-build.yaml`](.github/workflows/pr-windows-build.yaml)
+workflow can then be invoked in your branch via the Actions tab on your forked
+repository to build your patch.
+
+This workflow will not run automatically on pull requests opened against the
+`fluent/fluent-bit` repository so it's best you run it yourself in your fork. 
+
+The resulting binaries are uploaded as a Github workflow artifact which can be
+found on the workflow run page.
+
+At time of writing this workflow *does not run any of the Fluent Bit test suite*
+- so it only checks that the target branch can be compiled. To run tests, a local
+build will be required.
+
+#### Using Docker for Windows
+
+For Windows users with Hyper-V capable machines the simplest way to build a
+branch of Fluent Bit is to use
+[Docker Desktop for Windows](https://docs.docker.com/desktop/install/windows-install/)
+to build
+[`./dockerfiles/Dockerfile.windows`](./dockerfiles/Dockerfile.windows).
+
+This method does not require manual installation of any build dependencies.
+
+The Dockerfile build does *not* run any Fluent Bit test suites. To run tests you
+will need to use a different method.
+
+Most OS virtualisation tools and virtual machine hosting services do not enable
+nested virtualisation, so it is generally not possible to use this method on
+virtualized windows machines.
+
+#### Locally on a Windows machine
+
+Local compilation on a Windows machine takes the longest to set up, but is the
+most convenient for iterating development work.
+
+At time of writing this is the only way to run the Fluent Bit tests on Windows.
+
+In lieu of full Windows dev environment setup instructions, see these CI automation
+resources for how to set up a Windows build of fluent-bit:
+
+* [`dockerfiles/Dockerfile.windows`](./dockerfiles/Dockerfile.windows) - only build-able using Docker for Windows
+* [`appveyor.yml`](./appveyor.yml)
+* [`.github/workflows/call-build-windows.yaml`](.github/workflows/call-build-windows.yaml) - github automation that runs the build on a Windows worker.
+
+The dependencies must be present:
+
+* Microsoft Visual Studio C/C++ toolchain. The CI automation uses MSVC 2019 at time of writing. MSVC Community Edition works fine.
+* [CMake](https://cmake.org/) 3.x on the `PATH`
+* A build of [OpenSSL](https://www.openssl.org/) as static libraries, pointed to by the `-DOPENSSL_ROOT_DIR` CMake variable. The CI automation uses [Chocolatey](https://chocolatey.org/) to `choco install -y openssl`.
+* `flex.exe` and `bison.exe` must be present on the `PATH`. The CI automation uses https://github.com/lexxmark/winflexbison.
+
+Assuming that `cmake` is on the `PATH`, Visual Studio is installed,
+WinFlexBison is present at `%SYSTEMDRIVE%\WinFlexBison` and OpenSSL is present
+at `%PROGRAMFILES%\OpenSSL-Win64`, the following Powershell commands should
+produce a basic Fluent Bit build:
+
+```powershell
+$env:PATH += "$env:SystemDrive\WinFlexBison"
+mkdir build
+cd build
+cmake -DOPENSSL_ROOT_DIR="$env:ProgramFiles\OpenSSL-Win64" -S .. -B .
+cmake --build .
+```
+
+The build output will be `bin\Debug\fluent-bit.exe`.
+
+If in doubt, check the CI and build automation files referenced above for specifics.
+
 ### Valgrind
 
 [Valgrind](https://valgrind.org/) is a tool that will help you detect and diagnose memory issues in your code. It will check for memory leaks and invalid memory accesses.