You should make your own theme following the instructions here:
Your qa-theme.php file should override the head_custom() function, to include the question content in the header. The example below would include the whole question content:
class qa_html_theme extends qa_html_theme_base
$this->output('<META NAME="Description" CONTENT="'.strip_tags(@$this->content['q_view']['content']).'">');
If you want to just get the first few words, you can use qa_string_to_words() in qa-util-string.php, then array_slice() to get the first few words, then implode() to concatenate them back into the original string.