Add support docs

Author: rjfarmer

README.md

@@ -20,6 +20,8 @@ or install via pypi
 python -m pip install --upgrade --user gfort2py
 ````
 
+For a full list of supportetd platforms [see the support documentation](docs/support.md).
+
 ## Why use this over other Fortran to Python translators?
 
 gfort2py has three main aims:

docs/support.md

@@ -0,0 +1,39 @@
+# Supported platforms
+
+Here we describe the support available for different platforms, where platform means the combination of the CPU architecture, OS, and Python interpreter. Support for new platforms is welcome, but will be limited by what can be run on a Github action or other free CI testing system.
+
+
+## Level 1 Support
+
+These platforms are fully supported and all features should work. We test for all combinations of supported Python and gfortran versions.
+
+- x86/Linux/CPython
+- x86/MacOS/CPython
+- x86/Windows/CPython (via Choco)
+
+
+## Level 2 Support
+
+These platforms are fully supported and all features should work. However we only test a single supported Python and gfortran version.
+
+- x86/Linux/PyPy
+- x86/Windows/CPython (via Cygwin)
+- arm32v7/Linux/CPython
+- riscv64/Linux/CPython
+
+## Level 3 Support
+
+These platforms are partially supported and not all features work. We only test a single supported Python and gfortran version. To find unsupported features search the test for the given error message.
+
+- s390x/Linux/CPython ('Skip on big endian systems')
+- ppc64le/Linux/CPython ('Skip on ppc64le systems')
+
+# Unsuporrted platforms
+
+No platforms are currently unsupoorted.
+
+# Planned platforms
+
+These paaltforms may work but we have not yet gotten the test suite running on these platforms. But support is planned for the future.
+
+- M*-chips/MacOS/CPython