Document workon alias behavior with Projects plugin

[bpeterso2000]

The purpose of this change is to help users understand how to get workonbehaving similar to virtualenvwrapper'sworkonwhen the compat_aliasesandprojects` plugins are used together.

• The compat_aliases plug-in aliases workon to vf activate.
• The project plugin aliases workon to vf workon, but only when VIRTUALFISH_COMPAT_ALIASES is set when the plug-in is loaded.

So the order the two plugins are listed (i.e., loaded) matters, and so does whether or not the VIRTUALFISH_COMPAT_ALIASES environment variable is set.

If VIRTUALFISH_COMPAT_ALIASES is not set before the projects plugin is loaded or the compat_aliases is loaded after the project plugin then workon will activate the virtual environtment, but will not automatically change to the project directory.

See issue #137

[justinmayer]

Thank you for the thorough description of the current situation as well as the clear motivation for the contributed documentation enhancement. Much appreciated!