Enhanced "Example Usage" section (#435)

+added documentation on how to use the retrieved secrets
+ added an example of converting the vault-action outputs to json
* fix e2e test setup

integrationTests/e2e/setup.js

@@ -1,7 +1,7 @@
 const got = require('got');
 
 const vaultUrl = `${process.env.VAULT_HOST}:${process.env.VAULT_PORT}`;
-const vaultToken = `${process.env.VAULT_TOKEN || 'testtoken'}`
+const vaultToken = `${process.env.VAULT_TOKEN}` === undefined ? `${process.env.VAULT_TOKEN}` : "testtoken";
 
 (async () => {
     try {