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.
Don't access kvs directly but use x_module and field_data mechanisms
Create new structure allows override of usage id and category
location mapper maturation
richer exceptions, add_map return course_id, fix a bug, filter out unwanted kwargs in init
split get_item uses loc_mapper to decode old style Locations
Begin transactional with_version impl
Change create_item to optionally add the item to the head version and not create a new version.
Replace date lib w/ simpler tzAbbr
PdfViewer: Added missing link to compatibility.js
fix a11y error in help modal
edx.org/LMS: revises graphic-based login and registration banner text to render with system text
LMS startup needs to initialize the modulestores so that we don't lazy load all of the XML courses on first HTTP request
fix focus management for help modal
add ESC key functionality to forgot password modal
Add the ability to lock assets.
ui changes from design team feedback on studio assets page
Update upload test for what actually works now.
addressing PR sass/html comments on studio assets