[no-ci] Update README.md

README.md

@@ -134,6 +134,16 @@ The `roaring` library allows you to go back and forth between compressed Roaring
 ```
 
 
+### Goroutine safety
+
+In general, it not safe to access
+the same BitSet using different goroutines--they are
+unsynchronized for performance. Should you want to access
+a BitSet from more than one goroutine, you should
+provide synchronization. Typically this is done by using channels to pass
+the *BitSet around (in Go style; so there is only ever one owner),
+or by using `sync.Mutex` to serialize operations on BitSets.
+
 ## Implementation Note
 
 Go 1.9 introduced a native `math/bits` library. We provide backward compatibility to Go 1.7, which might be removed.