chore: add backers file

Author: heywhy

BACKERS.md

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+<h1 align="center">Sponsors &amp; Backers</h1>
+
+Elasticlunr is an MIT-licensed open source project. It's an independent project with its ongoing development made possible entirely thanks to the support by these awesome [backers](https://github.com/heywhy/ex_elasticlunr/blob/dev/BACKERS.md). If you'd like to join them, please consider:
+
+- [Become a backer or sponsor on Patreon](https://www.patreon.com/atandarash).