Github uses markdown for it's .md readme's - the one that you typically see rendered as html at bottom of a github project page. Consequently, bolding a text is as simple as wrapping the text using two asterisks **Bold text here**, which will produce a text enclosed with the strong tag - ie: bolding the text.
You can read about more markdown syntax's here: http://daringfireball.net/projects/markdown/syntax#em