Added MPL license badge to README.md

[tom-halpin]

Proposed changes

Describe the proposed changes. This is your opportunity to communicate to the maintainers of this repository as to why we should accept your PR. If your PR fixes a bug or resolves a feature request please include a link to the associated issue.

Types of changes

What types of changes does your code introduce?

Put an x in the boxes that apply

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bugfix (non-breaking change which fixes an issue)
• Code style update (formatting, renaming)
• Documentation update (if none of the other choices apply)
• New feature (non-breaking change which adds functionality)

Checklist

Put an x in the boxes that apply you can view the checklist as a heads up as to what we are going to look for before merging your code.

• I have read the CONTRIBUTING.md guidelines for this repository.
• I have included any and all necessary documentation where appropriate.
• I will fix any problems reported with this PR by the GitHub Actions on this repository.

Further comments

Feel free to to explain in more detail why you chose the solution you did including what if any alternatives you considered where applicable.