Add documentation for slide view mode to features page
Add documentation for slide view mode to features page

Since it's a very useful feature, we should mention it in multiple 
locations.

So we mention it in the slide mode section of the features page.

Signed-off-by: Sheogorath <1c2647ce2ee8a4ec8e5c8b1b245ca38f94844bab@shivering-isles.com>

jackycute/HackMD,hackmdio/hackmd,jackycute/HackMD,hackmdio/hackmd,hackmdio/hackmd,Yukaii/hackmd,jackycute/HackMD,Yukaii/hackmd,Yukaii/hackmd