add build instructions

README.md

@@ -16,3 +16,8 @@ really, you should use trickle.
 
 quickly slapping together an `LD_PRELOAD`able library that puts custom logic
 on top of an existing function was fun to make though.
+
+```
+zig build
+env LD_PRELOAD=zig-out/lib/libgeneric_socket_ratelimiter.so curl amogus
+```