descriptor: Define the 'sha256' algo identifier

Before this commit, there wasn't something obvious to point to if you
wanted to explain the sha256 identifier.

The "SHOULD be submitted" wording follows runtime-spec's example [1].

[1]: https://github.com/opencontainers/runtime-spec/blob/v1.0.0-rc4/config.md#platform

Signed-off-by: W. Trevor King <wking@tremily.us>

descriptor.md

@@ -76,9 +76,9 @@ hex         := /[a-f0-9]+/
 
 Some example digest strings include the following:
 
-digest                                                                            | description                                   |
-----------------------------------------------------------------------------------|------------------------------------------------
-sha256:6c3c624b58dbbcd3c0dd82b4c53f04194d1247c6eebdaab7c610cf7d66709b3b           | Common sha256 based digest                    |
+digest                                                                  | algorithm           |
+------------------------------------------------------------------------|---------------------|
+sha256:6c3c624b58dbbcd3c0dd82b4c53f04194d1247c6eebdaab7c610cf7d66709b3b | [SHA-256](#sha-256) |
 
 Before consuming content targeted by a descriptor from untrusted sources, the byte content SHOULD be verified against the digest.
 Before calculating the digest, the size of the content SHOULD be verified to reduce hash collision space.
@@ -109,6 +109,16 @@ D == ID(C) == '<alg>:' + EncodeHex(H(C))
 
 The _digest_ is confirmed as the content identifier by independently calculating the _digest_.
 
+#### Registered identifiers
+
+The following algorithm identifiers are defined by this specification:
+
+| identifier | algorithm           |
+|------------|---------------------|
+| `sha256`   | [SHA-256](#sha-256) |
+
+If a useful algorithm is not included in the above table, it SHOULD be submitted to this specification for standardization.
+
 #### SHA-256
 
 [SHA-256](https://tools.ietf.org/html/rfc4634#page-7) is a collision-resistant hash function, chosen for ubiquity, reasonable size and secure characteristics.