Markdown

Markdown is a plain text formatting syntax designed so that it can optionally be converted to HTML using a tool by the same name. Markdown is popularly used as format for readme files.
- from wikipedia

Markdown syntax is very straightforward using few characters (# = - * `) to format plain text files. It supports headers, links, images, blockquotes, lists, paragraphs, italics and bold etc formats.

To make a phrase italic in Markdown, you can surround words with an underscore
To make a phrase bold in Markdown, you can surround words with two asterisks
To make headers in Markdown, you preface the phrase with a hash mark (#), up to 6 # for h1 to h6
To create a block quote, all you have to do is preface a line with the "greater than" caret (>).
Use new line for a hard break; and use two spaces for a soft break.

There are many “extended” implementations of Markdown that support formats like tables, definition lists, footnotes, and more. However, they’re non-standard, so they’re not essential to learning the basics

For learning markdown,  http://markdowntutorial.com/ is a very good online tutorial to learn the basics.

There are two online tools very useful to write markdown
http://www.markdownviewer.com/
http://hashify.me/