Add documentation on platform specific checks

Intro

=====

This directory contains a few sets of files that are used for

configuration in diverse ways:

    *.conf      Target platform configurations, please read

                'Configurations of OpenSSL target platforms' for more

                information.

    *.tmpl      Build file templates, please read 'Build-file

                programming with the "unified" build system' as well

                as 'Build info files' for more information.

    *.pm        Helper scripts / modules for the main `Configure`

                script.  See 'Configure helper scripts for more

                information.

Configurations of OpenSSL target platforms

==========================================

      "";       # Make sure no lingering values end up in the Makefile

    -}

Configure helper scripts

========================

Configure uses helper scripts in this directory:

Checker scripts

---------------

These scripts are per platform family, to check the integrity of the

tools used for configuration and building.  The checker script used is

either {build_platform}-{build_file}-checker.pm or

{build_platform}-checker.pm, where {build_platform} is the second

'build_scheme' list element from the configuration target data, and

{build_file} is 'build_file' from the same target data.

If the check succeeds, the script is expected to end with a non-zero

expression.  If the check fails, the script can end with a zero, or

with a `die`.