[flang] Parallel runtime library design doc (PRIF) (#76088)

The Parallel Runtime Interface for Fortran (PRIF) specifies the interface design for 
supporting Fortran's multi-image parallel features in flang.

---------

Co-authored-by: Katherine Rasmussen <[email protected]>
Co-authored-by: Damian Rouson <[email protected]>
Co-authored-by: Dan Bonachea <[email protected]>

Author: 4 people

flang/docs/ParallelMultiImageFortranRuntime.md

@@ -0,0 +1,18 @@
+<!--===- docs/ParallelMultiImageFortranRuntime.md
+  
+   Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+   See https://llvm.org/LICENSE.txt for license information.
+   SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+  
+-->
+
+# Multi-Image Parallel Fortran Runtime
+
+
+The Parallel Runtime Interface for Fortran (PRIF) defines an 
+interface designed for LLVM Flang to target implementations of 
+Fortran's multi-image parallel features.
+
+The current revision of the PRIF specification is here:
+<https://doi.org/10.25344/S4CG6G>
+

flang/docs/index.md

@@ -78,6 +78,7 @@ on how to get in touch with us and to learn more about the current status.
    OpenMP-semantics
    OptionComparison
    Overview
+   ParallelMultiImageFortranRuntime
    ParameterizedDerivedTypes
    ParserCombinators
    Parsing