docs: add release note mention of platforms no longer built

relnotes/v2.0.md

@@ -25,3 +25,14 @@
 - Multiple connections are now used by default between v2 devices. The new
   default value is to use three connections: one for index metadata and two
   for data exchange.
+
+- The following platforms unfortunately no longer get prebuilt binaries for
+  download at syncthing.net and on GitHub, due to complexities related to
+  cross compilation with SQLite:
+
+  - dragonfly/amd64
+  - illumos/amd64 and solaris/amd64
+  - linux/ppc64
+  - netbsd/*
+  - openbsd/386 and openbsd/arm
+  - windows/arm