docs: restore GitHub badges to README (#1443)

## Summary
- Restores the 5 GitHub badges (PyPI, Python, License, CI, Docs) that
were accidentally removed when PR #1072 merged with a stale README base,
3 days after they were originally added in PR #1002.

## Test plan
- [ ] Verify badges render correctly on the GitHub README

Signed-off-by: Chris Wing <cwing@nvidia.com>

Author: cwing-nvidia

README.md

@@ -1,5 +1,11 @@
 # NeMo Gym
 
+[![PyPI](https://img.shields.io/pypi/v/nemo-gym)](https://pypi.org/project/nemo-gym/)
+[![Python](https://img.shields.io/pypi/pyversions/nemo-gym)](https://pypi.org/project/nemo-gym/)
+[![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
+[![CI](https://github.com/NVIDIA-NeMo/Gym/actions/workflows/unit-tests.yml/badge.svg)](https://github.com/NVIDIA-NeMo/Gym/actions/workflows/unit-tests.yml)
+[![Docs](https://img.shields.io/badge/docs-NVIDIA-brightgreen)](https://docs.nvidia.com/nemo/gym/main/about/)
+
 **[Requirements](#-requirements)** • **[Quick Start](#-quick-start)** • **[Available Environments](#-available-environments)** • **[Documentation & Resources](#-documentation--resources)** • **[Community & Support](#-community--support)** • **[Citations](#-citations)**
 
 NeMo Gym is a library for evaluating and improving models and agents using environments. NeMo Gym provides infrastructure to develop environments, scalably run evaluation and training, and a collection of popular benchmarks and training environments.