Created Middleware regex (markdown)

Middleware-regex.md

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+## WIP (pending PR#495)
+
+S3Proxy can use regex to rename blobs before they are uploaded to the backend. The regex middleware is configured as:
+
+```
+s3proxy.regex-blobstore.match.<regex name 1> = <regex match expression>
+s3proxy.regex-blobstore.replace.<regex name 1> = <regex replace expression>
+
+    .... [snip] ....
+
+s3proxy.regex-blobstore.match.<regex name N> = <regex match expression>
+s3proxy.regex-blobstore.replace.<regex name N> = <regex replace expression>
+```
+
+You can use multiple regex, and they are evaluated in order. The first regex that matches breaks the evaluation.
+Each regex match config MUST be accompanied by its replace rule.
+
+For example, the following rule:
+
+```
+s3proxy.regex-blobstore.match.type_partition=^prefix/(\\w+)/(\\d{4})/(\\d{2})/(\\d{2})/(.*)$
+s3proxy.regex-blobstore.replace.type_partition=prefix/date=$2-$3-$4/type=$1/$5
+```
+
+will rewrite blob name from:
+
+```
+prefix/test/2023/01/01/test.txt
+```
+
+to:
+
+```
+prefix/date=2023-01-01/type=test/test.txt
+```
+
+**NOTE:** Note that the match expression must be in escaped string format so, for example, in order to match a digit, you need to escape the backslash (e.g. use \\d NOT \d).