Added Derived Gates and Modified basic gates along with explanation, symbols, examples and Truth Tables.

[SiriVennela24-star]

In the Boolean Algebra Readme file, I had added a brief explanation of basic gates like 'and', 'or' and 'not'. Also added a brief explanation of derived gates like 'NAND', 'NOR', 'XOR' and 'XNOR'.

Concepts of logic gates are same. but I have added the exact defination, symbols, truth table and examples for clear understanding of all logic gates.

When the user opens the readme file, he/she can have a crystal-clear understanding of all basic and derived logic gates. Also with the help of examples, they can have a stong base of understanding too.