added backers & sponsors

[piamancini]

Thanks for taking the effort to create a PR!

If you are creating an extensive issue, you might want to open an issue with your idea first, so that you don't put a lot of effort in an PR that wouldn't be accepted ;-).

PR checklist:

• Implementation
• Added tests
• Verified that there is no significant performance drop (npm run perf)
• Updated (either in the description of this PR as markdown, or as seperate PR on the gh-pages branch. Please refer to this PR). For new functionality, at least API.md should be updated
• Added type annotations (if you are unfamiliar with typescript, untyped, :any based PR's are welcome as well

now your OpenCollective backers&sponsors will appear directly on the readme.
See how it will look: https://github.com/apex/apex#backers
deets: https://github.com/opencollective/opencollective/wiki/Github-banner
Added some badges @ the top as well. :)

[coveralls]

Coverage remained the same at 96.012% when pulling 2f65d98 on piamancini:patch-1 into 70fbdaa on mobxjs:master.