Define immutable tags

A tag MAY be recreated to refer a diferrent content.
However, a tag with "-immutable" suffix, such as "1.2.3-immutable", SHOULD not be recreated with a different content.
(Not "MUST not", because a registry implementation might not be aware of the history of the tag)

Closes issue 241

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

spec.md

@@ -508,6 +508,12 @@ To delete a blob, perform a `DELETE` request to a path in the following format:
 Upon success, the registry MUST respond with code `202 Accepted`.
 If the blob is not found, a `404 Not Found` code MUST be returned.
 
+##### Mutating tags
+
+A tag MAY be recreated to refer to a diferrent content.
+
+However, a tag with `-immutable` suffix, such as `1.2.3-immutable`, SHOULD not be recreated with a different content.
+
 ### API
 
 The API operates over HTTP. Below is a summary of the endpoints used by the API.