Readme improvement

[yigitozkavci]

• Cropped some screenshots. They were taking so much space, making it hard to reach the actual parts.
• Added table of contents:
  • Installation
  • Getting Started
  • Features
  • TODO
  • Contributing
  • License
• Added code examples to the Getting Started section.
• Grouped Low memory usage and Notification in Features section.
• Added contributing instructions.
• Added license link below.
• Renamed LICENCE to LICENSE, like GNU GENERAL LICENSE uses.

A note: if my changes are too marginal for the project, please tell me so that I can modify it accordingly.

[yasintoy]

Thank you so much. You've done a great job.