[chore] Add readme for nfs directory (#3012)

Author: thompson-tomo

docs/nfs/README.md

@@ -0,0 +1,13 @@
+<!--- Hugo front matter used to generate the website version of this page:
+linkTitle: NFS
+--->
+
+# Semantic conventions for NFS
+
+**Status**: [Development][DocumentStatus]
+
+Semantic conventions for NFS are defined for the following signals:
+
+* [NFS Metrics](nfs-metrics.md)
+
+[DocumentStatus]: https://opentelemetry.io/docs/specs/otel/document-status