capnproto: Add package

pkgs/development/libraries/capnproto/default.nix

@@ -0,0 +1,23 @@
+{ stdenv, fetchurl }:
+
+stdenv.mkDerivation {
+  name = "capnproto-0.4.1";
+
+  meta = with stdenv.lib; {
+    homepage    = "http://kentonv.github.io/capnproto";
+    description = "Cap'n Proto cerealization protocol";
+    longDescription = ''
+      Cap’n Proto is an insanely fast data interchange format and
+      capability-based RPC system. Think JSON, except binary. Or think Protocol
+      Buffers, except faster.
+    '';
+    license     = licenses.bsd2;
+    platforms   = platforms.all;
+    maintainers = with maintainers; [ cstrahan ];
+  };
+
+  src = fetchurl {
+    url = "https://capnproto.org/capnproto-c++-0.4.1.tar.gz";
+    sha256 = "8453e8d508906062f113dbdfff552f41e08083ccf7c9407778a8d107675cd468";
+  };
+}