Need to add minimum documentation on writing new models

[butler]

At a minimum the model editing documentation needs to be updated to reflect the new model structure and point to external web pages with information on how to write as well as to github repo of models that can be downloaded and used either as templates or to modify.

Eventually a more robust set of instructions would be useful. Suggest doing the minimum + for this release and closing this ticket. Once we know what we want we can open a new 4.1 ticket.

Note: there is some merit to using a web page, or better yet a wiki, to host the detailed instructions rather than bundling them. It makes them more easily updatable between releases. There must however be a solid set of build in instructions for when no internet connection is availble.

[ajj]

In 41e7f2e7bf3da407ccc23d81cdbc337a27eacecc/sasmodels:

Merge branch 'master' into ticket651

[ajj]

In 26d93689407c804e76d8b47e6be5dae7f47ceaa8/sasmodels:

Merge branch 'ticket651'