[doc] build-windows: Mention that only trusty works

doc/build-windows.md

@@ -6,6 +6,9 @@ Below are some notes on how to build Bitcoin Core for Windows.
 Most developers use cross-compilation from Ubuntu to build executables for
 Windows. This is also used to build the release binaries.
 
+Currently only building on Ubuntu Trusty 14.04 is supported.
+Other versions are unsupported or known to be broken (e.g. Ubuntu Xenial 16.04).
+
 While there are potentially a number of ways to build on Windows (for example using msys / mingw-w64),
 using the Windows Subsystem For Linux is the most straightforward. If you are building with
 another method, please contribute the instructions here for others who are running versions