add caveat about manually reformatting spreadsheets

[ErinBecker]

Episode 2 has a good exercise for fixing formatting issues in a badly formatted spreadsheet, however this may lead the learner to infer that manually formatting a spreadsheet is good practice (despite the fact that it's not reproducible). It would be good to note here the following (with a little exposition of each):

• This exercise is provided as an example of what good formatting looks like and why that formatting is needed.
• A note about reproducibility.
• It's best practice to format your data sheets like this from the beginning, but if you can't, at least save a raw version of your data before making any manual edits.
• Some note about how we'll be learning about OpenRefine for making more reproducible changes to data sheets later in the workshop.

[ErinBecker]

Closed in favor of solution proposed in #37.