README: add section with clearer compatibility and development steps

[MSevey]

Overview

Added a section to the README to make it more explicit as to which versions rollkit is compatible with with celestia and how to use the local devnet vs arabica and mocha.

Checklist

• New and updated code has appropriate documentation
• New and updated code has new and/or updated testing
• Required CI checks are passing
• Visual proof for any user facing features like CLI or documentation updates
• Linked issues closed with keywords

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.15 ⚠️

Comparison is base (c695de5) 54.92% compared to head (373abac) 54.78%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1011      +/-   ##
==========================================
- Coverage   54.92%   54.78%   -0.15%     
==========================================
  Files          60       60              
  Lines        6380     6380              
==========================================
- Hits         3504     3495       -9     
- Misses       2523     2531       +8     
- Partials      353      354       +1     

see 3 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.