Change docs regarding sending emails for custom events

[NicolasGorga]

Hello,

One of the problems I found with original plugin's docs, is the statement regarding sending emails for custom events. By reading the docs, one would assume that just by adding the custom event template key to plugin options, and corresponding template Sendgrid id as value, emails would be sent.

This alone won't fire an email, since the event is never registered against the NotificationProvider. I would suggest two alternatives:

1- Either change the README, indicating that apart from setting this configuration in plugin options, one needs to create a Subscriber and register the event against the Sendgrid provider, similar to order loader
2- Or change the loader code, to dynamically register the events taken from the plugiun options, against the sendgrid provider. This way, firing emails for custom events is as easy as firing them for the already defined ones, since you just add the config to plugin options and don't need to worry about creating the loader.

[luluhoc]

@NicolasGorga You can create a PR for the 1st option, cause I think someone should use the SendGrid service in their custom event rather than overcomplicate the settings.

[NicolasGorga]

Why do you see it as "overcomplicating the settings"? With the second option, the current behavior for plugin options would remain the same. Moreover, the end user wouldn't need to do an extra step of creating a loader for events that fall outside of the current default events registered by the plugin and that are also harcoded.

With this solution one would only need to define the keys in the plugin options as current, but not limited to a subset of events, but rather have access to all existent events + custom ones