doc: add supported platforms description

Closes: https://github.com/libuv/libuv/issues/983
PR-URL: https://github.com/libuv/libuv/pull/991
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

README.md

@@ -224,18 +224,7 @@ Run:
 
 ## Supported Platforms
 
-Microsoft Windows operating systems since Windows XP SP2. It can be built
+Check the [SUPPORTED_PLATFORMS file](SUPPORTED_PLATFORMS.md).
-with either Visual Studio or MinGW. Consider using
-[Visual Studio Express 2010][] or later if you do not have a full Visual
-Studio license.
-
-Linux using the GCC toolchain.
-
-OS X using the GCC or XCode toolchain.
-
-Solaris 121 and later using GCC toolchain.
-
-AIX 6 and later using GCC toolchain (see notes).
 
 ### AIX Notes
 
@@ -254,7 +243,6 @@ See the [guidelines for contributing][].
 [node.js]: http://nodejs.org/
 [GYP]: http://code.google.com/p/gyp/
 [Python]: https://www.python.org/downloads/
-[Visual Studio Express 2010]: http://www.microsoft.com/visualstudio/eng/products/visual-studio-2010-express
 [guidelines for contributing]: https://github.com/libuv/libuv/blob/master/CONTRIBUTING.md
 [libuv_banner]: https://raw.githubusercontent.com/libuv/libuv/master/img/banner.png
 [x32]: https://en.wikipedia.org/wiki/X32_ABI

SUPPORTED_PLATFORMS.md

@@ -0,0 +1,70 @@
+# Supported platforms
+
+|  System | Support type | Supported versions | Notes |
+|---|---|---|---|
+| GNU/Linux | Tier 1 | Linux >= 2.6.18 with glibc >= 2.5 | |
+| macOS | Tier 1 | macOS >= 10.7 | |
+| Windows | Tier 1 | Windows >= XP SP1 | MSVC 2008 and later are supported |
+| FreeBSD | Tier 1 | >= 9 (see note) | |
+| AIX | Tier 2 | >= 6 | Maintainers: @libuv/aix |
+| z/OS | Tier 2 | >= V2R2 | Maintainers: @libuv/zos |
+| Linux with musl | Tier 2 | musl >= 1.0 | |
+| SunOS | Tier 2 | Solaris 121 and later | Maintainers: @libuv/sunos |
+| MinGW | Tier 3 | MinGW32 and MinGW-w64 | |
+| Other | Tier 3 | N/A | |
+
+#### Note on FreeBSD 9
+
+While FreeBSD is supported as Tier 1, FreeBSD 9 will get Tier 2 support until
+it reaches end of life, in December 2016.
+
+## Support types
+
+* **Tier 1**: Officially supported and tested with CI. Any contributed patch
+  MUST NOT break such systems. These are supported by @libuv/collaborators.
+
+* **Tier 2**: Officially supported, but not necessarily tested with CI. These
+  systems are maintained to the best of @libuv/collaborators ability,
+  without being a top priority.
+
+* **Tier 3**: Community maintained. These systems may inadvertently break and the
+  community and interested parties are expected to help with the maintenance.
+
+## Adding support for a new platform
+
+**IMPORTANT**: Before attempting to add support for a new platform please open
+an issue about it for discussion.
+
+### Unix
+
+I/O handling is abstracted by an internal `uv__io_t` handle. The new platform
+will need to implement some of the functions, the prototypes are in
+``src/unix/internal.h``.
+
+If the new platform requires extra fields for any handle structure, create a
+new include file in ``include/`` with the name ``uv-theplatform.h`` and add
+the appropriate defines there.
+
+All functionality related to the new platform must be implemented in its own
+file inside ``src/unix/`` unless it's already done in a common file, in which
+case adding an `ifdef` is fine.
+
+Two build systems are supported: autotools and GYP. Ideally both need to be
+supported, but if GYP does not support the new platform it can be left out.
+
+### Windows
+
+Windows is treated as a single platform, so adding support for a new platform
+would mean adding support for a new version.
+
+Compilation and runtime must succeed for the minimum supported version. If a
+new API is to be used, it must be done optionally, only in supported versions.
+
+### Common
+
+Some common notes when adding support for new platforms:
+
+* Generally libuv tries to avoid compile time checks. Do not add any to the
+  autotools based build system or use version checking macros.
+  Dynamically load functions and symbols if they are not supported by the
+  minimum supported version.