Add line of explanation

Author: hannah-bulmer

package.json

@@ -17,7 +17,7 @@
     },
     "dependencies": {
         "@elastic.io/component-commons-library": "0.0.6",
-        "chai": "^4.2.0",
+        "chai": "4.2.0",
         "elasticio-node": "0.0.8",
         "elasticio-sailor-nodejs": "2.5.1",
         "mocha": "6.2.2",

spec/createPet.spec.js

@@ -14,6 +14,7 @@ const msg = {
   },
 };
 
+// we use sinon to mock functions such as `emit`, which come from sailor when the code is loaded onto the platform
 const self = {
   emit: sinon.spy(),
 };