Note that sets are optional

[treyhunner]

After the exercises for dictionaries, let's mention that typing out the sets section is optional and we suggest that students sit back and relax because. Sets are great to know about, but you can always look up how they work when you need them.

[jkrooskos]

I did pull request #24. I have a branch called origin/part4. I am not sure how to push to the same part4 branch Rise used.

[jkrooskos]

I deleted #24. #26 is the PR.