Provide friendlier message for missing consumer startup

[jchristgit]

Currently we emit the following message when no consumer was detected:

No consumers were running nor did any start up in time for shard session
startup. Is a consumer started as part of your supervision tree?

While this isn't bad, I think we would benefit from making this a bit more
user-friendly. For instance, people that are new to Elixir might not know how
to start the consumer as part of the supervision tree, or might have skipped
the docs for it by accident. Reading the README, we also currently only show
that you need to 1. install nostrum and 2. configure a bot token, but no
mention is made of having to configure the consumer. (I will open a separate
ticket for this)

We should update this message to be more clear and point to the README or
another example on how to properly start the consumer. For example:

No consumers were running nor did any start up in time for shard session startup.
nostrum's consumers receive events from the Discord Gateway, if no consumer is running, your bot will not receive any events.
Make sure that your application is starting a consumer as part of your supervision tree.
The documentation explains how to do this: https://hexdocs.pm/nostrum/intro.html#setup
If you're still unsure, feel free to ask for help on Discord: https://discord.gg/2Bgn8nW

Would be happy for other thoughts on this.