Add note about limitation of the plugin.

[gipi]

Clarify subtle limitation of this plugin as noted in #116

[aklajnert]

Thank you for your contribution. I think that this note should be put into README.rst, in the end of the Usage section. That way it will be also visible on GitHub. Perhaps, there should be two examples - good and bad one. Also, I think it will work with from subprocess import Popen in some cases, but it may not be reliable

[gipi]

I think what it's most disorienting it's that the examples are all testing directly the Popen() interface, when in the actual use case you test something using it internally.

I'll rework the PR to include some good and bad examples, hopefully before next week.