Created Middleware user metadata replacer (markdown)

Middleware-user-metadata-replacer.md

@@ -0,0 +1,7 @@
+Some object stores like Azure support a [limited set of user metadata names and values](https://learn.microsoft.com/en-us/rest/api/storageservices/naming-and-referencing-containers--blobs--and-metadata#metadata-key-and-value-names), e.g., no hyphens.  This middleware allows replacing the characters during putBlob and getBlob so that the client can interact with objects without code changes.  Configuration:
+
+```
+s3proxy.user-metadata-replacer-blobstore=true
+s3proxy.user-metadata-replacer-blobstore.from-chars=_
+s3proxy.user-metadata-replacer-blobstore.to-chars=-
+```