Improve wording of SDX readme.

[feamster]

There are a few things which could be worded better in HW7.

1. "First, you will configure the topology as shown in the figure. You will need two files:"
   There are more than "two files" which need to be edited. Saying there are only two files of interest is misleading. Also you might want to say "you will need to edit" not just "you will need"

$ sudo sh clean.sh
The instructions don't mention where to find it, it is not in the path, nor is it in the working directories in the home directory. Perhaps change it to:
sudo sh /vagrant/assignments/sdn-ixp/clean.sh

1. The policy section talks about HTTP and HTTPS. You do show examples of testing with port 80.
   But you don't mention port 443 (default HTTPS port) ANYWHERE in the document. Nor do you mention port 4321 ANYWHERE in the Assignment section. I think maybe you want us to continue working with port 4321? But if so, then why are you referring to HTTPS and not port 4321? Hmm. Maybe you really mean HTTP and HTTPS like you say. Then perhaps, simply add one more line that makes it clear that we can remove any references to port 4321 and port 4322 in the existing files.
   I do see that submit.py only tests port 80, so it isn't very concerned either way with the other ports, but it would be nice if the instructions were easier to interpret the intent.

[feamster]

Also, there's an error in the readme that has students run pyretic.py in the background. We should remove the "&".

[sairammohandass]

I would like to work on this issue can you please assign this to me?