Recommend that YAML keys be sorted by "spec order".

This commit adds a recommendation that

1) the keys in the YAML metadata block (in both "embedded" and
   "external" modes) SHOULD be sorted in the same order as the
    corresponding slots are listed in the definition of the MappingSet
    class in the SSSOM model;

2) the keys in the 'curie_map' dictionary (i.e. the prefix names) SHOULD
   be sorted by alphabetical order.

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Next
 
 - Add issue_tracker_item and issue_tracker [model elements](https://github.com/mapping-commons/sssom/pull/259).
+- Add recommendation to sort the keys in the YAML metadata block.
 
 ## SSSOM version 0.13.0
 

src/docs/spec.md

@@ -358,6 +358,8 @@ Note that the following prefixes are built-in and (1) MUST NOT be changed from t
 (The "canonical order" corresponds to the exact order of elements as seen in the specification.)
 This precludes spurious diffs in a git setting, which is an important concern for the continuous reviewing of mappings by curators and users. 
 
+**Canonical ordering of keys in the YAML metadata block**. For the same reason (avoiding spurious diffs), the keys in the YAML metadata block SHOULD be sorted in the same order as they are listed in the [MappingSet specification](https://mapping-commons.github.io/sssom/MappingSet/); keys in the `curie_map` dictionary SHOULD be sorted by alphabetical order. Those recommendations apply whether the metadata block is embedded in the TSV file or kept in a separate file.
+
 Note that only metadata elements permissible in a global context (G, or L/G) can be used in the metadata-file.
 
 We recommend to use the following *filename conventions* for SSSOM metadatafiles: