[fuzzing] document `npm run fuzz` in readme

Readme.md

@@ -14,6 +14,10 @@ To run the tests, execute the following commands:
     npm install
     npm test
 
+You can also run fuzzing tests (using [Echidna][echidna]) on the contracts:
+
+    npm run fuzz
+
 To start a local Ethereum node with the contracts deployed, execute:
 
     npm start
@@ -132,3 +136,4 @@ To Do
 
   * Analysis and optimization of gas usage
 
+[echidna]: https://github.com/crytic/echidna