Create calculating_offsets.md

[santosomar]

This pull request includes a detailed guide on calculating offsets for buffer overflows and explains why more data than the buffer size is often required in such exploits. The guide provides a step-by-step process and highlights key considerations for successful exploitation.

Key additions:

• buffer_overflow_example/calculating_offsets.md: Added a comprehensive guide on calculating offsets for buffer overflows, including steps like identifying vulnerable input points, pattern generation, causing program crashes, identifying offsets in crash data, and refining payloads based on offsets.
• Explained the necessity of sending more data than the buffer size to overwrite adjacent memory, reach control data, account for structure padding and alignment, and include NOP-sleds and shellcode.