Improve documentation

[btzr-io]

Add all features, and configurable options, also dev guide needs some changes.

[the-coder-dude]

@btzr-io what exactly i need to do?

[btzr-io]

@prateekpardeshi I'm planing to add a documentation section on readme file.
You can try to add a section like "Features" and add a list of the features of the viewer...

Features or stuff you can do etc..

• Navigation controls
• Fullscreen mode
  -...

Or if you can fix any typos you see and improve the formatting that would be great ✌️

[btzr-io]

Also a shortcut keys section would be useful to have 🤔