passkey : add short intro to README.md [no-ci] (#8317)

* passkey : add short intro to README.md [no-ci]

This commit adds a short introduction to the README.md file in the
examples/passkey directory.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

* Update examples/passkey/README.md

---------

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

examples/passkey/README.md

@@ -1,5 +1,8 @@
 # llama.cpp/example/passkey
 
+A passkey retrieval task is an evaluation method used to measure a language
+models ability to recall information from long contexts.
+
 See the following PRs for more info:
 
 - https://github.com/ggerganov/llama.cpp/pull/3856