Add out of date warning and footer 4 CAF_ABI.md

 [ci skip]

Author: zbeekman

CAF_ABI.md

@@ -24,6 +24,10 @@ standard-conforming Fortran 2008 or 2015 and compile their code with the OpenCoa
 The actual function names in this document have a PREFIX in the source code to avoid
 name clashes.  The prefix can be vendor-specific.
 
+### Warning ###
+
+*This document may be out of date.*
+
 To Do
 -----
 
@@ -560,6 +564,14 @@ __GCC__:
 __TODO__:
    Change `errmsg_len` to `size_t`
 
+---
+
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+[![Twitter URL](https://img.shields.io/twitter/url/http/shields.io.svg?style=social)](https://twitter.com/intent/tweet?hashtags=HPC,Fortran,PGAS&related=zbeekman,gnutools,HPCwire,HPC_Guru,hpcprogrammer,SciNetHPC,DegenerateConic,jeffdotscience,travisci&text=Stop%20programming%20w%2F%20the%20%23MPI%20docs%20in%20your%20lap%2C%20try%20Coarray%20Fortran%20w%2F%20OpenCoarrays%20%26%20GFortran!&url=https%3A//github.com/sourceryinstitute/opencoarrays)
+
+
 [Hyperlinks]:#
 
 [TS29113]: ftp://ftp.nag.co.uk/sc22wg5/n1901-n1950/n1942.pdf