Added FIM readme.

examples/fill-in-middle/README.md

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+
+# Example
+
+The FIM (Fill-In-Middle) objective is useful for generating text conditioned on a prefix and a suffix.
+This example is for use with codellama, for doing exactly that.
+
+For a quick summary of what's going on here, see issue #2818, and/or read [the FIM paper](https://arxiv.org/abs/2207.14255).
+
+```
+Usage: ./fill-in-middle <model> <prefix> <suffix> <n_max_tokens> <n_threads>
+```
+```sh
+./fill-in-middle \
+   CodeLlama-34B-GGUF/codellama-34b.Q4_K_S.gguf \
+   $'def add(a, b):\n' \
+   $'\n' \
+   64 \
+   4
+```
+
+With prefix:
+```py
+def add(a, b):
+
+```
+
+And a newline as suffix:
+```py
+
+```
+
+We can expect it to generate somethng like:
+```py
+    return a + b
+```