Update effective-forms.md

Replaced with a ton of content from "Designing transaction" in the Gov.uk Service Manual (https://www.gov.uk/service-manual/user-centred-design/designing-transactions.html), which uses the Open Government License v2.0 (http://www.nationalarchives.gov.uk/doc/open-government-licence/version/2/.
Update effective-forms.md

fixed image links
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Updated more links
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Added information about communicating errors
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content edits about validation and helping users
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Updated Table of Contents
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adjusted attribution language
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copy edits for line 24. not loving it yet.
Merge pull request #168 from benguhin/gh-pages

Update effective-forms.md
combined the anchor links about form structure

it's not necessary to separate "How to structure the form" from "Example structures" as jump links - I combined them as "Choosing a form structure"
Merging upstream / overriding my latest layout changes of Forms page with Ben's content updates.