rfc42: add subprocess protocol RFC

Problem: the subprocess execution protocol is not documented.

Add a new RFC.

Author: garlick

README.md

@@ -51,6 +51,7 @@ Table of Contents
 - [39/Flux Security Signature](spec_39.rst)
 - [40/Fluxion Resource Set Extension](spec_40.rst)
 - [41/Job Information Service](spec_41.rst)
+- [42/Subprocess Server Protocol](spec_42.rst)
 
 Build Instructions
 ------------------

index.rst

@@ -272,6 +272,13 @@ to store resource graph data in RFC 20 *R* version 1 objects.
 The Flux Job Information Service provides proxy access to KVS job
 information for guest users.
 
+:doc:`spec_42`
+~~~~~~~~~~~~~~
+
+The subprocess server protocol is used for execution, monitoring, and
+standard I/O management of remote processes.
+
+
 .. Each file must appear in a toctree
 .. toctree::
    :hidden:
@@ -315,3 +322,4 @@ information for guest users.
    spec_39
    spec_40
    spec_41
+   spec_42