Add system setup details for static preview

MathB uses tools like pandoc and convert to display static
preview of the rendered code for clients where JavaScript is
disabled. These tools need to be installed for static preview to
work correctly. Add the details about the packages to be
installed in the README file.
Display preview button when JavaScript is disabled

When JavaScript is disabled, the user cannot see a live preview
of the rendered code. A preview button can help the user to see
a static preview (rendered with an image) before he or she saves
the post. The preview button should be displayed if and only if
JavaScript is disabled.