rfc25: add references for dependencies and constraints

Problem: Both dependencies and constraints are supported in Jobspec
V1, but they are not mentioned in RFC 25.

Add RFC 26 and RFC 31 to the list of Related Standards and dependencies
and constraints to the list of common `system` keys to make this
support explicit and easy to search.

Author: grondo

spec_25.rst

@@ -38,6 +38,9 @@ Related Standards
 
 -  :doc:`20/Resource Set Specification Version 1 <spec_20>`
 
+-  :doc:`26/Job Dependency Specification <spec_26>`
+
+-  :doc:`31/Job Constraints Specification <spec_31>`
 
 Goals
 -----
@@ -172,6 +175,10 @@ definitions can be found in RFC14. Values MAY have any valid YAML type.
 
    -  cwd
 
+   -  dependencies
+
+   -  constraints
+
 Most system attributes are optional, but the ``duration`` attribute is required in
 jobspec V1.