Expand on "Goroutine safety" section of README

It would be great if the Goroutine Safety section of the README.md about what operations are currently goroutine-safe could be expanded on, even/especially if that's not a guarantee that they will remain thread-safe moving forward.

For example, if bitmap rb1 is created, optimized, and never written to again (or has appropriate locking), can it be safely used in concurrent read operations by another bitmap rb2.And(rb1) or rb2.Or(rb1)?

Testing this with go test -race (which I know isn't a anywhere near complete test) doesn't report any issues:

func TestConcurrentBitmapAccess(t *testing.T) {
	rb1 := roaring.BitmapOf(1, 20, 40, 90)

	var wg sync.WaitGroup
	for i := 0; i < 1000; i++ {
		wg.Add(1)
		go func() {
			rb2 := roaring.NewBitmap()
			rb2.And(rb1)
			wg.Done()
		}()
	}
	wg.Wait()
}

Start with the Goroutine Safety section in README.md and inspect the bitmap operations relevant to the rb2.And(rb1) and rb2.Or(rb1) examples. Run the supplied concurrency example with go test -race, then document which operations are currently safe, including the no-write or locking conditions and whether these are guarantees.