When knitting a github_document latex equations are not rendered correctly

While working on a github readme file I noticed that when I set as output github_document the latex doesn't render correctly. E.g the following Rmd file:

output: github_document

$$ y = a \cdot x $$

is rendered as

[ y = a \cdot x ]


output: html_document

$$ y = a \cdot x $$

is correctly rendered as


I am working with RStudio 1.3.1093 and was wonderign if I am not using correctly the github_document option

github_document will output a markdown file using GFM markup syntax. It is a text file so you won't get the same output than html_document.
Using github_document a preview is generated - this preview tries to be close to what you can see on Github (like in a README). And github does not support math rendering.

You won't get Math in Github Readme without using a trick