Document how to set credentials

[jtpio]

It would be useful to add some documentation for setting custom credentials, when using a released version of jupydrive-s3 (not local).

Currently it looks like there is some logic relying in the use of process.env here:

jupydrive-s3/src/index.ts

Lines 93 to 104 in 7b79ecf

	bucket: process.env.JP_S3_BUCKET ?? 'jupyter-drives-test-bucket-1',
	root: process.env.JP_S3_ROOT ?? '',
	config: {
	forcePathStyle: true,
	endpoint: process.env.JP_S3_ENDPOINT ?? 'https://example.com/s3',
	region: process.env.JP_S3_REGION ?? 'eu-west-1',
	credentials: {
	accessKeyId:
	process.env.JP_S3_ACCESS_KEY_ID ?? 'abcdefghijklmnopqrstuvwxyz',
	secretAccessKey:
	process.env.JP_S3_SECRET_ACCESS_KEY ??
	'SECRET123456789abcdefghijklmnopqrstuvwxyz'

[cesnikb]

I'm having issues configuring credentials on released version as well. Documentation for it would come in handy.