Markdown formatting fixes
Markdown formatting fixes

Blank lines around headers, spaces after pound signs in headers. The old version doesn't render properly on Github.
pleroy/Principia,pleroy/Principia,pleroy/Principia,mockingbirdnest/Principia,mockingbirdnest/Principia,mockingbirdnest/Principia,eggrobin/Principia,pleroy/Principia,eggrobin/Principia,eggrobin/Principia,mockingbirdnest/Principia