Delete / Cleanup Branches Not Following New Naming Convention, Add New Contribution Guidelines

[pwdel]

New Contribution Guidelines should be added here:

https://github.com/openpredictionmarkets/socialpredict/blob/main/CONTRIBUTING.md

This document should be linked from the main README.md in an appropriate table of content format.

Specifically, we need to communicate the new standard for branch naming. E.g. under point 3...

3. **Create a Branch**
   - Create a new branch in your local repository before starting your work.
   - Use a clear branch name that describes the intent of your work, such as `feature/add-login` or `fix/header-bug`.

However, we must include the allowed branch names.

To make this documentation evergreen, we could just point it to the workflows file on our backend:

socialpredict/.github/workflows/backend.yml

Line 39 in cfd7a0a

startsWith(github.head_ref, 'feature/')

Currently, the following branches are allowed:

    if: |
      startsWith(github.head_ref, 'feature/')
      || startsWith(github.head_ref, 'fix/')
      || startsWith(github.head_ref, 'refactor/')
    runs-on: ubuntu-latest

That is:

feature, fix, refactor