Created Middleware alias blobstore (markdown)

Middleware-alias-blobstore.md

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+S3Proxy can use an alias for a backend container. This may be helpful to preserve bucket names when switching backends, for example. The alternate name can be specified as follows:
+```
+s3proxy.alias-blobstore.content=backend-data
+```
+
+In the above example, requests for the bucket `content` are redirected to `backend-data`. This does not preclude using the name `backend-data`, however, when listing buckets `backend-data` bucket is replaced with `content` in the listing.
+
+S3Proxy does not allow mapping multiple aliases to the same backend bucket. Future work could expand this middleware to include a prefix, allowing a subset of a bucket to appear as a bucket to the S3Proxy clients.