Add GH Funding Metadata

[SheetJSDev]

With this change, GH will show a Sponsor button at the top. Here's how GitHub described the feature:

Open source projects can also express their funding models directly from their repositories. When .github/FUNDING.yml is added to a project’s master branch, a new “Sponsor” button will appear at the top of the repository. Clicking the button opens a natively rendered view of the funding models listed in that file.

[jsf-clabot]

All committers have signed the CLA.

[coveralls]

Coverage increased (+0.07%) to 92.808% when pulling 34fc8fe on SheetJSDev:patch-1 into b9fbd69 on mochajs:master.

[boneskull]

thanks!