Updated README.md

[inobelar]

Changes:

• Added link to IV (Initialization Vector) - useful for people, dont familiar with it
• One 'iv' abbreviation capitalized (iv -> IV)
• Link to 'Tiny AES in C' library not show with its name
• Note: Use it at your own risk! changed to GitHub's blockqute syntax
• For code blocks added shell mark - for better highlighting
• For "counter mode" and "cyber block chaining mode" added "(CTR)" and "(CBC)" descriptions