provide installation instructions for models in the model marketplace

[pkienzle]

Each model page should include instructions telling the user to place the model files into .sasview/plugin_models in their home directory, and where to find the new plugin model in the sasview model menu. Also note that the plugins.log file in this directory will contain any errors that arise when the model is run. These instructions could be automatically added to each page, either directly or using a link to the marketplace section in the sasview manual.

Placing a model in the plugin directory means that sasview will run arbitrary C and python code downloaded from the web. Users should be very strongly encouraged to examine the source when downloading to be sure it doesn't contain malware.

[pkienzle]

On mac the dot-directories are not visible in Finder, so the docs need to include the following:

On MacOS X click on the "home" folder, and then press "Shift + Command + ." 
to make ~/.sasview available in Finder

Maybe we should put the files in ~/Library/Application Support/SasView rather than ~/.sasview when on the Mac so that it is easier for users to find them?