Add a note to README

[SamTheGeek]

Adding a note to the section of the readme about fetching new tweets, noting that your account must be publicly visible since the bearer token can only see public tweets. This caused me some heartache when trying to use this functionality

[nhoizey]

@SamTheGeek would you have a link to Twitter's documentation where we can read that the bearer token only allows getting public tweets?

[zachleat]

I would also add a note about the limitation that images cannot be fetched (via eleventy-img) from protected accounts!

[SamTheGeek]

@nhoizey It's on the Oauth 2.0 page, under bearer token.

This method is typically for developers that need read-only access to public information.

Elsewhere on the developer site, the app-only methods are described as

App only Access Token allows a Twitter developer app to access information publicly available on Twitter.

[nhoizey]

@SamTheGeek could you please add the link to https://developer.twitter.com/en/docs/authentication/oauth-2-0 in your PR, and the additional mention of images as suggested by @zachleat, so I can merge?