Link to How To Contribute wiki page

Created a CONTRIBUTING.rst which GitHub will pick up and included a
link to the How To Contribute wiki page there. Also added the same link
to the README in the "How To Contribute" section.
Moved to a more comprehensive contributing doc.

Based on the How To Contribute wiki page (and switched to markdown so
it's a simple copy-paste).
Link to wiki with absolute URLs
Added linebreaks.
Use comments to skip tests instead of a tag
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With tests, some settings changes
(all should default to not breaking anything for edx)

Added styling for shopping cart User Experience
- Styled shoppingcart list page
- Styled navigation shopping cart button
- Styled receipt page
- Styled course about page for shopping cart courses

Addressed HTML/SCSS issues

Remove offending body class and unnecessary sass changes

Addresses many review comments on stanford shopping cart

* framework for generating order instructions on receipt page
  in shoppingcart.models
* move user_cart_has_item into shoppingcart.models
* move min_course_price_for_currency into course_modes.models
* remove auto activation on purchase
* 2-space indents in templates
* etc

revert indentation on navigation.html for ease of review

pep8 pylint

move logging/error handling from shoppingcart view to model

Addressing @dave changes
Merge pull request #1005 from edx/jtauber/link-to-how-to-contribute

Include / Link to How To Contribute wiki page
Merge pull request #1143 from edx/will/skip-tests-with-comments

Use comments to skip tests instead of a tag
Merge pull request #1118 from edx/jbau/shoppingcart-stanford

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Pull XModule attributes out into a mixin that can be applied to xblocks