Minor readme fixes

[mruegenberg]

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• [x ] I've followed the contributing guidelines to the best of my understanding
• [ x] I've read and understood the contributing guidelines
• [ x] I've formatted my code according to Natron's code style
• [x ] I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

What type of PR is this? (Check one of the boxes below)

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x ] Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] My change requires a change to the documentation
  • [ ] I have updated the documentation accordingly

What does this pull request do?

Minor updates to the readme to reduce confusion for new contributors like me

Have you tested your changes (if applicable)? If so, how?

Not a code change, so nothing to test

Futher details of this pull request

• Removed Bountysource from the readme as it appears to be dead
• Changed the branch referenced from RB-2.4 to RB-2.5

[mruegenberg]

@devernay Github is telling me there's "1 workflow awaiting approval" and no option to merge the PR

[rodlie]

Sorry about that, PR's from new contributors needs approval before workflows can run. I approved the workflow and merged your changes.