docs: add README and LICENSE

Author: ivanvc

LICENSE

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+Copyright (c) 2023 Iván Valdés Castillo
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+<img src="https://github.com/ivanvc/android-grayscale-toggle/raw/main/app/src/main/ic_launcher-playstore.png"
+     width="15%" align="right" alt="Logo" style="padding-left: 10px" />
+
+# Android Grayscale Toggle
+
+This is a minimal and straightforward application to toggle the grayscale color
+correction setting on Android devices.
+
+If the application doesn't have permission to change system settings, it will
+show instructions on how to do it with ADB. Otherwise, launching the app toggles
+the setting. This application is helpful with launchers such as Niagara
+Launcher, where you can define an action for a home screen button, or with
+devices with a designated function physical button, like Unihertz phones.
+
+[Many](https://www.wired.com/story/grayscale-ios-android-smartphone-addiction/)
+[articles](https://observer.com/2018/05/grayscale-can-cure-smartphone-addiction/)
+[cover](https://www.theverge.com/23637672/grayscale-iphone-android-pixel-samsung-galaxy-how-to)
+[why](https://lifehacker.com/change-your-screen-to-grayscale-to-combat-phone-addicti-1795821843)
+[having](https://www.vice.com/en/article/xwm38k/grayscale-setting-phone-addiction)
+your smartphone screen in grayscale is a good idea. However, sometimes it's
+helpful to switch to full color in case you need to open a navigation app, or if
+you want to see a picture you got from a message, or if you're showing your
+phone screen to another person.
+
+## Installation
+
+Get the APK from the [releases page](https://github.com/ivanvc/android-grayscale-toggle/releases). Then, install it on your phone.
+
+## Granting Permission to the App
+
+1. Ensure you have ADB installed and configured on your machine. Please take a
+   look at the [official ABD page](https://developer.android.com/tools/adb).
+2. Connect your phone to your computer and allow USB debugging.
+3. Open a terminal and run:
+   ```sh
+   adb shell pm grant vc.ivan.grayscaletoggle android.permission.WRITE_SECURE_SETTINGS
+   ```
+
+That's it. Opening the app will toggle the grayscale mode.
+
+## License
+
+See [LICENSE](LICENSE) © [Ivan Valdes](https://github.com/ivanvc/)