Add Open Collective

Hi, I'm making updates for Open Collective. Either you or another core contributor signed this repository up for Open Collective. This pull request adds financial contributors from your Open Collective https://opencollective.com/assimp ❤️

  What it does:
  - adds a badge to show the latest number of financial contributors
  - adds a banner displaying contributors to the project on GitHub
  - adds a banner displaying all individuals contributing financially on Open Collective
  - adds a section displaying all organizations contributing financially on Open Collective, with their logo and a link to their website

P.S: As with any pull request, feel free to comment or suggest changes.

  Thank you for your great contribution to the Open Source community. You are awesome! 🙌
  And welcome to the Open Collective community! 😊

  Come chat with us in the #opensource channel on https://slack.opencollective.com - great place to ask questions and share best practices with other Open Source sustainers!

Readme.md

@@ -2,6 +2,7 @@ Open Asset Import Library (assimp)
 ==================================
 A library to import and export various 3d-model-formats including scene-post-processing to generate missing render data.
 ### Current project status ###
+[![Financial Contributors on Open Collective](https://opencollective.com/assimp/all/badge.svg?label=financial+contributors)](https://opencollective.com/assimp) 
 ![C/C++ CI](https://github.com/assimp/assimp/workflows/C/C++%20CI/badge.svg)
 [![Linux Build Status](https://travis-ci.org/assimp/assimp.svg)](https://travis-ci.org/assimp/assimp)
 [![Windows Build Status](https://ci.appveyor.com/api/projects/status/tmo433wax6u6cjp4?svg=true)](https://ci.appveyor.com/project/kimkulling/assimp)
@@ -179,6 +180,28 @@ And we also have a Gitter-channel:Gitter [![Join the chat at https://gitter.im/a
 Contributions to assimp are highly appreciated. The easiest way to get involved is to submit
 a pull request with your changes against the main repository's `master` branch.
 
+## Contributors
+
+### Code Contributors
+
+This project exists thanks to all the people who contribute. [[Contribute](CONTRIBUTING.md)].
+
+<a href="https://github.com/assimp/assimp/graphs/contributors"><img src="https://opencollective.com/assimp/contributors.svg?width=890&button=false" /></a>
+
+### Financial Contributors
+
+Become a financial contributor and help us sustain our community. [[Contribute](https://opencollective.com/assimp/contribute)]
+
+#### Individuals
+
+<a href="https://opencollective.com/assimp"><img src="https://opencollective.com/assimp/individuals.svg?width=890"></a>
+
+#### Organizations
+
+Support this project with your organization. Your logo will show up here with a link to your website. [[Contribute](https://opencollective.com/assimp/contribute)]
+
+<a href="https://opencollective.com/assimp/organization/0/website"><img src="https://opencollective.com/assimp/organization/0/avatar.svg"></a>
+
 ### License ###
 Our license is based on the modified, __3-clause BSD__-License.