Clarify how metadata is stored with the S3 backend (#223)

This was suggested in #221.

README.md

@@ -126,7 +126,7 @@ The following storage backends are available:
 |Name|Notes|Options
 |----|-----|-------
 |LocalFS|Enabled by default, this backend uses the filesystem|```filespath = files/``` -- Path to store uploads (default is files/)<br />```metapath = meta/``` -- Path to store information about uploads (default is meta/)|
-|S3|Use with any S3-compatible provider.<br> This implementation will stream files through the linx instance (every download will request and stream the file from the S3 bucket).<br><br>For high-traffic environments, one might consider using an external caching layer such as described [in this article](https://blog.sentry.io/2017/03/01/dodging-s3-downtime-with-nginx-and-haproxy.html).|```s3-endpoint = https://...``` -- S3 endpoint<br>```s3-region = us-east-1``` -- S3 region<br>```s3-bucket = mybucket``` -- S3 bucket to use for files and metadata<br>```s3-force-path-style = true``` (optional) -- force path-style addresing (e.g. https://<span></span>s3.amazonaws.com/linx/example.txt)<br><br>Environment variables to provide:<br>```AWS_ACCESS_KEY_ID``` -- the S3 access key<br>```AWS_SECRET_ACCESS_KEY ``` -- the S3 secret key<br>```AWS_SESSION_TOKEN``` (optional) -- the S3 session token|
+|S3|Use with any S3-compatible provider.<br> This implementation will stream files through the linx instance (every download will request and stream the file from the S3 bucket). File metadata will be stored as tags on the object in the bucket.<br><br>For high-traffic environments, one might consider using an external caching layer such as described [in this article](https://blog.sentry.io/2017/03/01/dodging-s3-downtime-with-nginx-and-haproxy.html).|```s3-endpoint = https://...``` -- S3 endpoint<br>```s3-region = us-east-1``` -- S3 region<br>```s3-bucket = mybucket``` -- S3 bucket to use for files and metadata<br>```s3-force-path-style = true``` (optional) -- force path-style addresing (e.g. https://<span></span>s3.amazonaws.com/linx/example.txt)<br><br>Environment variables to provide:<br>```AWS_ACCESS_KEY_ID``` -- the S3 access key<br>```AWS_SECRET_ACCESS_KEY ``` -- the S3 secret key<br>```AWS_SESSION_TOKEN``` (optional) -- the S3 session token|
 
 
 #### SSL with built-in server