Add NOTICE file (celestiaorg#1466)

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## Overview

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Based on some offline discussions, to properly use the LICENSE the repo
needs a NOTICE file that explains how to use the LICENSE.

This PR creates the minimal NOTICE file which identifies the repo name
and the copyright info.

## Checklist

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- [x] New and updated code has appropriate documentation
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- [x] Required CI checks are passing
- [x] Visual proof for any user facing features like CLI or
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- [x] Linked issues closed with keywords

NOTICE

@@ -0,0 +1,2 @@
+Celestia App
+Copyright 2021 and onwards Strange Loop Labs AG