Updated Storage backend compatibility (markdown)

Storage-backend-compatibility.md

@@ -7,6 +7,7 @@ Some storage backends do not support all the features of S3:
 ## Azure
 
 * limits container names to 63 characters and restricts character set to lowercase alphanumeric and dash
+* limits metadata keys to "C# identifiers". As far as I can tell, that's alphanumeric characters and dash; must not start with a number
 * does not support blob names with `\` or with sequences of `./` and `/.`
 * maximum multipart upload size of 195 GB, see [JCLOUDS-1223](https://issues.apache.org/jira/browse/JCLOUDS-1223)
 * does not support Expires header but does support Cache-Control header