Note the huge prefix/null byte gotcha in README

Fixes #37

Also fixes #36

README.md

@@ -15,6 +15,12 @@ in a more efficient manner than performing each operation one at a time.
 
 For a mutable variant, see [go-radix](https://github.com/armon/go-radix).
 
+## :warning: This tree implementation is low-level and can be easily used incorrectly
+   - **No key may be a prefix of any other.**
+     - This is _not_ enforced but _will cause undefined behavior_ if violated. 
+     - The typical way to work around this is to append a null byte (`0x0`) to each key unless you know in advance your keys will never be prefixes. We don't always do this in tests if we arranged them to not be prefixes anyway!
+     - We could avoid this gotcha by automatically adding null bytes to all keys but that has high cost of causing re-allocation of keys for every operation that the caller might not want if they already.
+
 Documentation
 =============
 
@@ -28,9 +34,9 @@ Below is a simple example of usage
 ```go
 // Create a tree
 r := iradix.New()
-r, _, _ = r.Insert([]byte("foo"), 1)
-r, _, _ = r.Insert([]byte("bar"), 2)
-r, _, _ = r.Insert([]byte("foobar"), 2)
+r, _, _ = r.Insert([]byte("foo\x00"), 1)
+r, _, _ = r.Insert([]byte("bar\x00"), 2)
+r, _, _ = r.Insert([]byte("foobar\x00"), 2)
 
 // Find the longest prefix match
 m, _, _ := r.Root().LongestPrefix([]byte("foozip"))
@@ -44,17 +50,17 @@ Here is an example of performing a range scan of the keys.
 ```go
 // Create a tree
 r := iradix.New()
-r, _, _ = r.Insert([]byte("001"), 1)
-r, _, _ = r.Insert([]byte("002"), 2)
-r, _, _ = r.Insert([]byte("005"), 5)
-r, _, _ = r.Insert([]byte("010"), 10)
-r, _, _ = r.Insert([]byte("100"), 10)
+r, _, _ = r.Insert([]byte("001\x00"), 1)
+r, _, _ = r.Insert([]byte("002\x00"), 2)
+r, _, _ = r.Insert([]byte("005\x00"), 5)
+r, _, _ = r.Insert([]byte("010\x00"), 10)
+r, _, _ = r.Insert([]byte("100\x00"), 10)
 
 // Range scan over the keys that sort lexicographically between [003, 050)
 it := r.Root().Iterator()
 it.SeekLowerBound([]byte("003"))
 for key, _, ok := it.Next(); ok; key, _, ok = it.Next() {
-  if key >= "050" {
+  if key >= "005" {
       break
   }
   fmt.Println(key)