Improve style in Markdown files

[jhlegarreta]

Current Markdown are hard to read. It would be nice if we increased consistency:

• Using dashes or asterisks for lists, but avoid using both.
• Typically, dashes under headings require the former to cover the entire length of the heading characters. Otherwise, the heading may not be displayed correctly. Changing them for prefix hashtags can be simpler.

Also, limiting the line length to e.g. 79 characters would greatly improve readability.