README.md missing GDI specification version badge

[pmcollins]

Summary

The README.md is missing a badge indicating which version of the GDI specification this distro supports.

Spec requirement

GDI Specification v1.8.0 — repository.md:

MUST have a badge on the README.md with GDI specification version supported

Current state

README.md has PyPI version and Python version badges but no GDI spec version badge.

Expected

Add a badge linking to the supported GDI spec release, e.g.:

[![GDI Specification](https://img.shields.io/badge/GDI%20specification-v1.8.0-blueviolet)](https://github.com/signalfx/gdi-specification/releases/tag/v1.8.0)

References

• README.md