Merge changes from default to to-markdown.

--HG--
branch : to-markdown
Convert formatting guide to Markdown.

This includes a number of content changes to account for the changed
presentation.

--HG--
branch : to-markdown
extra : rebase_source : c675dd5f48dc9a2b943e96e47757b254a8e72188
extra : amend_source : 6683e9d97ee8cffa082b30faf5ccbcde1dce2e14
welcome: Add legal and such sections; subsumes lawyer.html.
credit: Convert to Markdown, add credits for new contributors.

--HG--
branch : to-markdown
Convert crashe.html to Markdown.

--HG--
branch : to-markdown
Merge to-markdown into default.