userdefined machine method should be documented

[ekluzek]

Setting up a "userdefined" machine is no longer an option for cesm acme. The minimum that would need to be done to add this functionality would be to add a "userdefined" machine to config_machines.xml in either cesm or acme.

This is a useful option to have in order for people to run simple configurations for testing on their laptop, or for university users to get a start on configuring their own machine.

[jedwards4b]

We have an alternative method for this now. You can now put config_machines.xml, config_batch.xml and/or config_compilers.xml in a directory $HOME/.cime and they will be appended to the objects read from the cime_config directories. These files, once created, should continue to work across multiple cime versions. We will provide templates for these in documentation.

[billsacks]

Closing as a duplicate of #1293