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.
Make Textbooks properly lazy
Adding Pillow jpeg and tiff system libs
Replace date lib w/ simpler tzAbbr
Added tags to acceptance tests to define multiple shards.
Namespace the acceptance tests so they show up nicely in jenkins

Conflicts:
	cms/djangoapps/contentstore/features/advanced-settings.feature
	cms/djangoapps/contentstore/features/checklists.feature
	cms/djangoapps/contentstore/features/component.feature
	cms/djangoapps/contentstore/features/course-overview.feature
	cms/djangoapps/contentstore/features/course-settings.feature
	cms/djangoapps/contentstore/features/course-team.feature
	cms/djangoapps/contentstore/features/course-updates.feature
	cms/djangoapps/contentstore/features/courses.feature
	cms/djangoapps/contentstore/features/discussion-editor.feature
	cms/djangoapps/contentstore/features/grading.feature
	cms/djangoapps/contentstore/features/html-editor.feature
	cms/djangoapps/contentstore/features/problem-editor.feature
	cms/djangoapps/contentstore/features/section.feature
	cms/djangoapps/contentstore/features/signup.feature
	cms/djangoapps/contentstore/features/static-pages.feature
	cms/djangoapps/contentstore/features/subsection.feature
	cms/djangoapps/contentstore/features/textbooks.feature
	cms/djangoapps/contentstore/features/upload.feature
	cms/djangoapps/contentstore/features/video-editor.feature
	cms/djangoapps/contentstore/features/video.feature
	lms/djangoapps/courseware/features/certificates.feature
	lms/djangoapps/courseware/features/help.feature
	lms/djangoapps/courseware/features/high-level-tabs.feature
	lms/djangoapps/courseware/features/homepage.feature
	lms/djangoapps/courseware/features/login.feature
	lms/djangoapps/courseware/features/lti.feature
	lms/djangoapps/courseware/features/navigation.feature
	lms/djangoapps/courseware/features/problems.feature
	lms/djangoapps/courseware/features/registration.feature
	lms/djangoapps/courseware/features/signup.feature
	lms/djangoapps/courseware/features/video.feature
	lms/djangoapps/courseware/features/word_cloud.feature
Add "Download CSV of all student anonymized IDs" button to instructor dashboard

This is a recurrent ops problem, so we wanted to make it available
on the instructor dashboard.
Integrate split testing and LMS tabs experiments
Refactor tab tests
Fix for bug BLD-359: Unnecessary focus outline around Video Player.
PdfViewer: Added missing link to compatibility.js
Knock 10 more seconds off the acceptance tests.