Add libcni README that explains how it relates to the spec.

Nothing dramatic, but just to have a thing

Signed-off-by: Benjamin Leggett <[email protected]>

libcni/README.md

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+# libcni
+
+`libcni` is a library that parses and loads CNI configurations as defined in the [CNI spec](../SPEC.md).
+
+It is designed to be used by runtimes for this purpose, and is kept in sync with the CNI spec as a reference library implementation so that runtimes do not have to build and maintain their own implementations of the CNI spec, or construct their own parsing and loading logic.
+
+It is not required to use this library to be compliant with the [CNI spec](../SPEC.md).
+
+While the [CNI spec](../SPEC.md) only dictates the API and types, and does not dictate operational concerns or how or where from configuration is loaded, `libcni` is an opinionated, file-based implementation, and primarily loads and validates CNI spec-compliant configuration files from disk.