README: Contributions.

Adding new stuff is fine, though it NEEDS to be working on the
main target OSes. That's OpenBSD, Linux, FreeBSD and to some
degree macOS and DragonFlyBSD...If you want to add a new
OS, it'd be better if the patch included as much functionality
as possible :)

README

@@ -23,3 +23,9 @@ INSTALL:
 
 $ ninja -C build install
 
+CONTRIBUTIONS:
+
+Patches with bug fixes are more than welcome. If you do wish to
+add a substantial querying feature PLEASE ensure that it works reliaibly
+on OpenBSD, Linux and FreeBSD (ideally macOS also). UI changes
+should be platform independent.