ACME branched from CESM (cesm1_3_beta10) and the original README is below.

For details on the state of CESM source when ACME branched, retrieve
the ChangeLog and SVN_EXTERNAL_DIRECTORIES files listed below from the Git history
(a4394726d8a00481922d61d7d23bf04fb90dd81d or earlier).  

./ChangeLog
./SVN_EXTERNAL_DIRECTORIES

NOTE: The workflow described below is not used by ACME.
----------------------------------------------------------------------

       The Community Climate System Model version 4.0 (CCSM4.0)

                     http://www2.cesm.ucar.edu

       See the CCSM web site for documentation and information.


For commits to the ccsm svn repo

1) check out the latest ccsm4 tag but do not populate external directories
> svn co --ignore-externals $SVNREPO/ccsm4/tags/ccsm4_0_alpha## my_sandbox
> svn co -N $SVNREPO/ccsm4/tags/ccsm4_0_alpha## my_sandbox

2) modify the file SVN_EXTERNAL_DIRECTORIES with any changes to
component tags
> cd  my_sandbox
> emacs SVN_EXTERNAL_DIRECTORIES

3) set the property for the external definitions - don't forget the dot at the end
> svn propset  svn:externals  -F SVN_EXTERNAL_DIRECTORIES .

4) populate your sandbox with the external files
> svn update

5) test

6) document your mods
> cp  ChangeLog_template  ChangeLog.new
> cat  ChangeLog  >>  ChangeLog.new
> emacs  ChangeLog.new
> mv ChangeLog.new  ChangeLog

7) copy your sandbox to a new tag in the repository
> svn copy . $SVNREPO/ccsm4/tags/ccsm4_0_alpha## -m "created tag ccsm4_0_alpha##"