Help populating the examples README.md files

[ggerganov]

For now I just added empty README.md files:

• https://github.com/ggerganov/llama.cpp/tree/master/examples/main
• https://github.com/ggerganov/llama.cpp/tree/master/examples/quantize
• https://github.com/ggerganov/llama.cpp/tree/master/examples/perplexity
• https://github.com/ggerganov/llama.cpp/tree/master/examples/embedding
• etc.

It would be great to add usage instructions and various tips and tricks for better experience for each example.

Great task for initial contributions