Activating Open Collective

[xdamman]

This pull request adds backers and sponsors from your Open Collective https://opencollective.com/parse-server ❤️

It adds two badges at the top to show the latest number of backers and sponsors. It also adds placeholders so that the avatar/logo of new backers/sponsors can automatically be shown without having to update your README.md. [more info]

See how it looks on this repo.

We have also added a postinstall script to let people know after npm|yarn install that you are welcoming donations. [More info]

[codecov]

Codecov Report

Merging #3806 into master will decrease coverage by 2.37%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3806      +/-   ##
==========================================
- Coverage   92.49%   90.11%   -2.38%     
==========================================
  Files         118      114       -4     
  Lines        8246     7549     -697     
==========================================
- Hits         7627     6803     -824     
- Misses        619      746     +127

Impacted Files	Coverage Δ
src/Routers/CloudCodeRouter.js	42.85% <0%> (-57.15%)	⬇️
src/Routers/FilesRouter.js	48.11% <0%> (-50.8%)	⬇️
src/cli/definitions/parse-server.js	50% <0%> (-50%)	⬇️
src/Routers/RolesRouter.js	41.17% <0%> (-46.33%)	⬇️
src/Routers/InstallationsRouter.js	76.66% <0%> (-23.34%)	⬇️
src/Adapters/Files/GridStoreAdapter.js	85.71% <0%> (-14.29%)	⬇️
src/password.js	87.5% <0%> (-12.5%)	⬇️
src/Controllers/LiveQueryController.js	47.61% <0%> (-11.48%)	⬇️
src/Routers/SessionsRouter.js	85.36% <0%> (-8.39%)	⬇️
src/Adapters/Auth/vkontakte.js	26.66% <0%> (-6.67%)	⬇️
... and 49 more

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