Cleanup for identifier URL guidance re #398

Author: philipashlock

Diff for: metadata-changelog.md

@@ -31,6 +31,7 @@ Changes made to establish the [v1.1 Schema](/v1.1/schema)
 	* Adds `name` as part of `publisher` replacing earlier use of `publisher` ([#296](https://github.com/project-open-data/project-open-data.github.io/issues/296)) 
 	* Adds optional `subOrganizationOf` field as part of `publisher` to include parent organization hierarchy ([#296](https://github.com/project-open-data/project-open-data.github.io/issues/296)) 
 * Adds optional `@type` field to specify a JSON-LD data type ([#338](https://github.com/project-open-data/project-open-data.github.io/issues/388))
+* Recommends that `identifier` uses a URL, but does not require it. ([#69](https://github.com/project-open-data/project-open-data.github.io/issues/69))
 
 ##### Dataset Required-if-Applicable Fields  
 * Removes `webService` field. APIs can be described within a `distribution` using `accessURL` and `format` ([#291](https://github.com/project-open-data/project-open-data.github.io/issues/291)) 

Diff for: v1.1/schema.md

@@ -531,7 +531,7 @@ Dataset Fields {#Dataset}
 **Cardinality** | (1,1)
 **Required** | Yes, always
 **Accepted Values** | String
-**Usage Notes** | This field allows third parties to maintain a consistent record for datasets even if title or URLs are updated. Agencies may integrate an existing system for maintaining unique identifiers. Each identifier **must** be unique across the agency's catalog and remain fixed. Characters should be alphanumeric. It is **highly recommended** that a [URI (preferably an HTTP URL)](http://tools.ietf.org/html/rfc3986#section-1.1.3) be used to provide a globally unique identifier. Identifier URLs should be designed and maintained to persist indefinitely regardless of whether the URL of the resource itself changes.
+**Usage Notes** | This field allows third parties to maintain a consistent record for datasets even if title or URLs are updated. Agencies may integrate an existing system for maintaining unique identifiers. Each identifier **must** be unique across the agency's catalog and remain fixed. It is **highly recommended** that a [URI (preferably an HTTP URL)](http://tools.ietf.org/html/rfc3986#section-1.1.3) be used to provide a globally unique identifier. Identifier URLs should be designed and maintained to persist indefinitely regardless of whether the URL of the resource itself changes.
 **Example** |  `{"identifier":"http://dx.doi.org/10.7927/H4PZ56R2"}`
 
 {: .table .table-striped #isPartOf}