Readme body markup

[ericawright]

Status: Ready for Review Owner: @ericawright Reviewers: @mikenikles @fractaltheory @jansepar @marlowpayne @stewartyu

Changes

• clarify instructions
• remove references to 'modal-center' and 'sheet...'
• change default coverage to 80%
• change 'noop' to be valid code in examples
• clarify info on how to handle position fixed elements

  Issue:

https://github.com/mobify/pikabu/issues/51

Todos:

• [x] Engineer +1

  Feedback:

none so far

How to Test

• View README

[mikenikles]

:+1: from me. Let's wait for @fractaltheory or @jansepar to approve this as well.

[fractaltheory]

LGTM, :+1: