Mention segment setup in readme

[lispingit]

Similar to #2, I wasn't sure when/how to set up segment based on the readme. So initially, I tried the example code, which calls [SEGAnalytics sharedAnalytics], but this gave an error because SEGAnalytics wasn't configured yet.

I'd suggest adding segment to the list in the Requirements section, explaining that it needs to be configured with its own key, and linking to their docs, or giving example code like configureAnalytics.

[alfredp]

Updated https://github.com/Factual/segment-analytics-factual-engine-ios#requirements

[alfredp]

Our log in to www.segment.io is:

• user: demo@factual.com
• pass: Factual1

For a test app:

• Segment write key
  • ZKF2vMwVPJO7ri4KTEnm7HdFKIAtdWBU
• Engine API key for demo@factual.com
  • cYfImwkOWLW3TaKuQoCqwVqr912SAXwCQJpWe6Yk