update language on contact index page

this change:

* simplifies the first sentence
* moves the link to the help pages to the bottom, because data shows that few people
click on
* brings the description of the GOV.UK contact form closer in line with the User Support
team's actual remit
index page: language feedback from @jonsanger
Merge pull request #88 from alphagov/update-language-on-contact-index-page

update language on contact index page