annotations: make a designated doc and DRY a bit

In DRY'ing up the references to annotations. This makes the defined keys
in a very logical place rather than the bottom of one of the other docs.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

annotations.md

@@ -0,0 +1,23 @@
+# Annotations
+Several components of the specification, like [Image Manifests](manifest.md) and [Descriptors](descriptor.md), feature an optional annotations property, whose format is common and defined in this section.
+
+This property contains arbitrary metadata.
+
+## Rules
+
+Annotations MUST be a key-value map where both the key and value MUST be strings.
+While the value MUST be present, it MAY be an empty string.
+Keys MUST be unique within this map, and best practice is to namespace the keys.
+Keys SHOULD be named using a reverse domain notation - e.g. `com.example.myKey`.
+Keys using the `org.opencontainers` namespace are reserved and MUST NOT be used by other specifications and extensions.
+If there are no annotations then this property MUST either be absent or be an empty map.
+Consumers MUST NOT generate an error if they encounter an unknown annotation key.
+
+## Pre-Defined Annotation Keys
+
+This specification defines the following annotation keys, intended for but not limited to manifest list and image manifest authors:
+* **org.opencontainers.created** date on which the image was built (string, date-time as defined by [RFC 3339](https://tools.ietf.org/html/rfc3339#section-5.6)).
+* **org.opencontainers.authors** contact details of the people or organization responsible for the image (freeform string)
+* **org.opencontainers.homepage** URL to find more information on the image (string, a URL with scheme HTTP or HTTPS)
+* **org.opencontainers.documentation** URL to get documentation on the image (string, a URL with scheme HTTP or HTTPS)
+* **org.opencontainers.source** URL to get source code for the binary files in the image (string, a URL with scheme HTTP or HTTPS)

manifest-list.md

@@ -67,15 +67,9 @@ For the media type(s) that this document is compatible with, see the [matrix][ma
 - **`annotations`** *string-string map*
 
     This OPTIONAL property contains arbitrary metadata for the manifest list.
-    Annotations MUST be a key-value map where both the key and value MUST be strings.
-    While the value MUST be present, it MAY be an empty string.
-    Keys MUST be unique within this map, and best practice is to namespace the keys.
-    Keys SHOULD be named using a reverse domain notation - e.g. `com.example.myKey`.
-    Keys using the `org.opencontainers` namespace are reserved and MUST NOT be used by other specifications.
-    If there are no annotations then this property MUST either be absent or be an empty map.
-    Implementations that are reading/processing manifest lists MUST NOT generate an error if they encounter an unknown annotation key.
-
-    See [Pre-Defined Annotation Keys](manifest.md#pre-defined-annotation-keys).
+    This OPTIONAL property MUST use the [annotation rules](annotations.md#rules).
+
+    See [Pre-Defined Annotation Keys](annotations.md#pre-defined-annotation-keys).
 
 ### Extensibility
 Implementations that are reading/processing manifest lists MUST NOT generate an error if they encounter an unknown property.

manifest.md

@@ -62,15 +62,9 @@ Unlike the [Manifest List](manifest-list.md), which contains information about a
 - **`annotations`** *string-string map*
 
     This OPTIONAL property contains arbitrary metadata for the image manifest.
-    Annotations MUST be a key-value map where both the key and value MUST be strings.
-    While the value MUST be present, it MAY be an empty string.
-    Keys MUST be unique within this map, and best practice is to namespace the keys.
-    Keys SHOULD be named using a reverse domain notation - e.g. `com.example.myKey`.
-    Keys using the `org.opencontainers` namespace are reserved and MUST NOT be used by other specifications.
-    If there are no annotations then this property MUST either be absent or be an empty map.
-    Implementations that are reading/processing the image manifest MUST NOT generate an error if they encounter an unknown annotation key.
+    This OPTIONAL property MUST use the [annotation rules](annotations.md#rules).
 
-    See [Pre-Defined Annotation Keys](#pre-defined-annotation-keys).
+    See [Pre-Defined Annotation Keys](annotations.md#pre-defined-annotation-keys).
 
 ### Extensibility
 Implementations that are reading/processing image manifests MUST NOT generate an error if they encounter an unknown property.
@@ -111,11 +105,3 @@ Instead they MUST ignore unknown properties.
   }
 }
 ```
-
-# Pre-Defined Annotation Keys
-This specification defines the following annotation keys, which MAY be used by manifest list and image manifest authors:
-* **org.opencontainers.created** date on which the image was built (string, date-time as defined by [RFC 3339](https://tools.ietf.org/html/rfc3339#section-5.6)).
-* **org.opencontainers.authors** contact details of the people or organization responsible for the image (freeform string)
-* **org.opencontainers.homepage** URL to find more information on the image (string, must be a URL with scheme HTTP or HTTPS)
-* **org.opencontainers.documentation** URL to get documentation on the image (string, must be a URL with scheme HTTP or HTTPS)
-* **org.opencontainers.source** URL to get source code for the binary files in the image (string, must be a URL with scheme HTTP or HTTPS)