examples : add readme files

examples/lookahead/README.md

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+# llama.cpp/examples/lookahead
+
+Demonstartion of lookahead decoding technique:
+
+https://lmsys.org/blog/2023-11-21-lookahead-decoding/
+
+More info: https://github.com/ggerganov/llama.cpp/pull/4207

examples/speculative/README.md

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+# llama.cpp/examples/speculative
+
+Demonstartion of speculative decoding and tree-based speculative decoding techniques
+
+More info:
+
+- https://github.com/ggerganov/llama.cpp/pull/2926
+- https://github.com/ggerganov/llama.cpp/pull/3624