added some documentation for S3_UPLOADS_CACHE_CONTROL and S3_UPLOADS_EXPIRES constants

```

Note: as either `<from>` or `<to>` can be S3 or local locations, you must speficy the full S3 location via `s3://mybucket/mydirectory` for example `cp ./test.txt s3://mybucket/test.txt`.

Cache Control

==========

You can define the default HTTP `Cache-Control` header for uploaded media using the

following constant:

```PHP

define( 'S3_UPLOADS_CACHE_CONTROL', 30 * 24 * 60 * 60 );

	// will expire in 30 days time

```

You can also configure the `Expires` header using the `S3_UPLOADS_EXPIRES` constant

For instance if you wanted to set an asset to effectively not expire, you could

set the Expires header way off in the future.  For example:

```PHP

define( 'S3_UPLOADS_EXPIRES', gmdate( 'D, d M Y H:i:s', time() + (10 * 365 * 24 * 60 * 60) ) .' GMT' );

	// will expire in 10 years time

```