resolves #27 add SDR for standard form open block delimiter

This MR records the decision resulting from the proposal to change the open block delimiter form from -- to ~~~~. The SDR (Specification Decision Record) is a concise summary of the results of the discussion in the associated issue. It's not meant to record all the alternative proposals or all the supporting and detracting evidence surrounding the decision. Think of it as more of a pointer to the issue for people who want to dive deep into the reason behind an addition, removal, or change to AsciiDoc.