Update .env.example with helpful comments and documentation links.

.env.example

@@ -11,9 +11,11 @@ ECONUMO_CONNECT_USERS=true
 # MAILER_DSN=smtp://user:[email protected]:25
 
 # Uncomment this if you use multi currencies:
+# Check for more details: https://econumo.com/docs/self-hosting/multi-currency/
 # ECONUMO_CURRENCY_BASE=USD
 
 # To enable the System API (api/v1/system/) uncomment the following string (its disabled by default):
+# Check for more details: https://econumo.com/docs/api/
 # ECONUMO_SYSTEM_API_KEY=REPLACE_WITH_RANDOM_STRING
 ###< Econumo app ###