Update README on building openmpi

[estepona]

What

Building openmpi requires having the submodules initiated, otherwise it would error out. This PR simply updates the README regarding this issue.

Example

...
Open MPI autogen (buckle up!)

1. Checking tool versions

   Searching for autoconf
     Found autoconf version 2.71; checking version...
       Found version component 2 -- need 2
       Found version component 71 -- need 69
     ==> ACCEPTED
   Searching for libtoolize
     Found libtoolize version 2.4.6; checking version...
       Found version component 2 -- need 2
       Found version component 4 -- need 4
       Found version component 6 -- need 2

     ==> ACCEPTED
   Searching for automake
     Found automake version 1.16.5; checking version...
       Found version component 1 -- need 1
       Found version component 16 -- need 13
     ==> ACCEPTED

2. Checking for git submodules

    ==> ERROR: Missing submodule

The submodule "config/oac" is missing.

Ref: https://github.com/open-mpi/ompi/blob/main/autogen.pl#L1397-L1477

[swx-jenkins3]

Can one of the admins verify this patch?