Add README to maxtext.trainers

[melissawm]

Description

Adds a README to the trainers submodule.

This was created with the help of Gemini to verify/check information.

Tests

N/A

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive README.md for the maxtext.trainers package, detailing its layout, entry points (pretraining, DiLoCo, post-training, tokenizer), and related documentation. Feedback was provided to correct the documentation URLs in the post-training table from posttraining to post_training to maintain consistency with standard naming conventions and prevent potential broken links.