document proxy setting

FWIW - proxy setup can be complex - not sure that it makes sense to give
step by step directions (especially given the different proxy options
that exist), but this commit notes the types of settings to worry about.

doc/sphinx-guides/source/installation/config.rst

@@ -541,6 +541,7 @@ dataverse.files.<id>.url-expiration-minutes  <?>                 If direct uploa
 dataverse.files.<id>.min-part-size           <?>                 Multipart direct uploads will occur for files larger than this. Optional.  ``1024**3``
 dataverse.files.<id>.custom-endpoint-url     <?>                 Use custom S3 endpoint. Needs URL either with or without protocol.         (none)
 dataverse.files.<id>.custom-endpoint-region  <?>                 Only used when using custom endpoint. Optional.                            ``dataverse``
+dataverse.files.<id>.proxy-url               <?>                 URL of a proxy protecting the S3 store. Optional.                          (none)
 dataverse.files.<id>.path-style-access       ``true``/``false``  Use path style buckets instead of subdomains. Optional.                    ``false``
 dataverse.files.<id>.payload-signing         ``true``/``false``  Enable payload signing. Optional                                           ``false``
 dataverse.files.<id>.chunked-encoding        ``true``/``false``  Disable chunked encoding. Optional                                         ``true``
@@ -559,6 +560,9 @@ Reported Working S3-Compatible Storage
   Set ``dataverse.files.<id>.payload-signing=true`` and ``dataverse.files.<id>.chunked-encoding=false`` to use Surf Object
   Store.
 
+Note that the ``dataverse.files.<id>.proxy-url`` setting can be used in installations where the object store is proxied. For direct uploads and downloads, Dataverse redirects to the proxy-url but presigns the urls using the ``dataverse.files.<id>.custom-endpoint-url``.
+Additional configuration (appropriate CORS settings, proxy caching/timeout configuration, and proxy settings to pass headers to/from S3 and to avoid adding additional headers) will also be needed to enable use of a proxy with direct upload and download.
+
 **HINT:** If you are successfully using an S3 storage implementation not yet listed above, please feel free to
 `open an issue at Github <https://github.com/IQSS/dataverse/issues/new>`_ and describe your setup.
 We will be glad to add it here.