rfc31: add RFC 31: Job Constraints Specification

Problem: There is no defintion of how to encode job constraints in
a standardized format.

Introduce RFC 31: Job Constraints Specification, which includes a simple,
but extensible format for encoding job constraints, inspired by the
json-logic definition at https://jsonlogic.com.

README.md

@@ -41,6 +41,7 @@ Table of Contents
 - [28/Flux Resource Acquisition Protocol Version 1](spec_28.rst)
 - [29/Hostlist Format](spec_29.rst)
 - [30/Job Urgency](spec_30.rst)
+- [31/Job Constraints Specification](spec_31.rst)
 
 Build Instructions
 ------------------

index.rst

@@ -249,3 +249,4 @@ This specification describes the Flux job urgency parameter.
    spec_28
    spec_29
    spec_30
+   spec_31

spec_31.rst

@@ -71,9 +71,12 @@ supported
    SHALL indicate a property that MUST NOT be included in the allocated
    resource set.
 
+<<<<<<< HEAD
 Future extensions
 ~~~~~~~~~~~~~~~~~
 
+=======
+>>>>>>> 18c8696... rfc31: add RFC 31: Job Constraints Specification
 The following constraint operators MAY be supported. If a job is submitted
 using these constraint operators, and the operators are not supported by
 the instance, then the job SHALL be rejected with an appropriate error
@@ -97,7 +100,21 @@ message:
       { "or": [{ "properties": [ "foo" ]}, { "properties": [ "bar" ]}] }
 
    
+<<<<<<< HEAD
  - ``and``: Simple logical ``and``.
+=======
+ - ``and``: Simple logical ``and``. Evaluates to true only if all ``value``
+   arguments are true, e.g.
+
+   .. code:: json
+
+      { "and":
+        [ {"properties": [ "a" ] },
+          { "not": [{ "properties": [ "foo", "bar" ]}
+        ]
+      }
+     
+>>>>>>> 18c8696... rfc31: add RFC 31: Job Constraints Specification
 
 Examples
 --------