README.md: update doc with a note on OS support

Mention that we support all of Linux/MacOS/Windows, but that the JIT
does not work on Windows (see issue #21 about that).

README.md

@@ -35,6 +35,9 @@ a disassembler.
 It is based on Rich Lane's [uBPF software](https://github.com/iovisor/ubpf/),
 which does nearly the same, but is written in C.
 
+The crate is supposed to compile and run on Linux, MacOS X, and Windows,
+although the JIT-compiler does not work with Windows at this time.
+
 ## Link to the crate
 
 This crate is available from [crates.io](https://crates.io/crates/rbpf), so it