add attack part of bep126

BEP126.md

@@ -177,6 +177,10 @@ who didn’t vote for the block or vote later won’t get reward.
 * Once the malicious validator has been put into jail, the later submitters that submit malicious evidence of the
   validators won’t get any reward.
 
+### 5.5 Stopping Attacks
+There are two well-known attacks against proof-of stake system: *long range revisions* and *catastrophic crashes*, these attacks
+are also existed after implementing this BEP.
+
 ## 6. License
 
 The content is licensed under [CC0](https://creativecommons.org/publicdomain/zero/1.0/).