message: docs: add note about the build matrix to the CONTRIBUTING.md docs [skip ci]
diff: +# Contributing<!-- omit in toc -->
  +
  +- [Contributing with Issues](#contributing-with-issues)
  +- [Contributing with Code](#contributing-with-code)
  + - [C/C++](#cc)
  + - [Code Style](#code-style)
  + - [Linting](#linting)
  + - [Typescript / Javascript](#typescript--javascript)
  + - [Code Style](#code-style-1)
  + - [Linting](#linting-1)
  + - [Setup](#setup)
  + - [Adding New libcurl Options](#adding-new-libcurl-options)
  + - [Changing libcurl Version Used on Prebuilt Binaries for Windows](#changing-libcurl-version-used-on-prebuilt-binaries-for-windows)
  + - [Debugging with lldb](#debugging-with-lldb)
  + - [Publishing New Releases](#publishing-new-releases)
  + - [Semver Major / Minor / Patch](#semver-major--minor--patch)
  + - [Prereleases](#prereleases)
  + - [Build Matrix](#build-matrix)
  +
  ## Contributing with Issues
  Before opening an issue try to search the existing ones for the same problem.
  @@ -146,3 +165,28 @@ If for some reason np fails to run with Yarn, you can use this command to skip c
  ```shell
  $ yarn np prerelease --no-yarn --no-cleanup --any-branch --tag next
  ```
  +
  +#### Build Matrix
  +
  +We are using three CI providers:
  +- CircleCI
  +- GitHub Actions
  +- AppVeyor
  +
  +Each CI provider is responsible for some builds:
  +
  +CircleCI:
  +- Node.js (Alpine)
  +- Electron (linux)
  +- NW.js (linux)
  +
  +GitHub Actions:
  +- Node.js (Linux, macOS)
  +- Electron (macOS)
  +- NW.js (macOS)
  +
  +AppVeyor:
  +- Node.js (Win64, Win32)
  +- Electron (Win64, Win32)
  +
  +GitHub Actions are also used to lint PRs, for that a build runs on Linux.
  