added easy-to-find setup instructions for a local development environment

[dhanushsaimudari]

PR Description

Summary

Addresses #149 by adding a streamlined "Development Setup" section to the README.md.

Based on maintainer feedback, this version:

• Prioritizes the Standard Installation method for general users.
• De-duplicates instructions by providing direct links to the official SunPy Conda Guide.
• Provides a concise 3-minute workflow for contributors to set up an editable environment and verify it via pytest.

Related Issues

Closes #149

Checklist

• I have added a entry to the changelog/ directory (if required).
• My PR description follows the official template.
• I have verified the installation steps on Windows 10.

AI Assistance Disclosure

AI tools were used for:

• Documentation (Drafting the initial structure and troubleshooting table)
• Research and understanding (Comparing rST syntax and formatting)

Verification Results (Windows 10)

I verified the setup manually with the following results:

python -c "import radiospectra; print(radiospectra.__version__)" 
# ✓ Output: 0.1.dev521+g91d3035

pytest tests/ -v --disable-warnings
# ✓ Result: Tests pass (pyparsing warnings noted as normal)

[samaloney]

Thanks for the PR just a few housekeeping issues please retain and use the PR template which is created when the PR is opened. (You can see it here: https://github.com/sunpy/.github/blob/main/.github/PULL_REQUEST_TEMPLATE.md)

Also this PR seems to contain a method from another pice or work or PR? I think the standard install should come before the dev install. Also a lot of this is already cover in the sunpy dev guide e.g. (https://docs.sunpy.org/en/latest/dev_guide/contents/conda_for_dependencies.html) should reference that as much as possible.

[dhanushsaimudari]

Extremely sorry for the mix-up! I accidentally pushed two different tasks into the same branch. I have now separated them into two distinct branches: one for the Documentation (#149) and another for the frequency_at_index feature. I will close the old PR and open two new clean ones