Document plugin options

[robinmetral]

Hi @jessestuart, thanks a lot for the great plugin!

Just a suggestion here: would you mind updating the README for the plugin options, namely this part:

{
  resolve: 'gatsby-source-s3-image',
  options: {
    bucketName: 'jesse.pics',
    domain: null, // [optional] Not necessary to define for AWS S3; defaults to `s3.amazonaws.com`
    protocol: 'https', // this defaults to https
  },
}

I've had to go look at the source to figure out essential options like region and accessKeyId.

Here they are, essentially:

gatsby-source-s3-image/src/source-nodes.ts

Lines 18 to 30 in 8d46555

	export interface SourceS3Options {
	// NOTE: Required params.
	accessKeyId: string
	secretAccessKey: string
	bucketName: string
	// Defaults to `${bucketName}.s3.amazonaws.com`, but may be overridden to
	// e.g., support CDN's (such as CloudFront), or any other S3-compliant API
	// (such as DigitalOcean Spaces.)
	domain?: string
	region?: string
	// Defaults to HTTP.
	protocol?: string
	}

And as far as I understand them:

• accessKeyId: not required if public, does the plugin work with a private bucket? See No nodes created by this plugin #332 ❓
• secretAccessKey: not required if public ❓
• bucketName: required ✔️
• domain: defaults to s3.amazonaws.com ✔️
• region: defaults to us-east-1, change for other regions ❌
• protocol: defaults to http (not https!), change for https ❌

Let me know if you'd prefer a PR, but I thought you might want to handle the README yourself 🙂

Cheers!