Delete dubious MIT license (#1100)

* Delete diubious MIT license

AO2 does not gain copyright by people adding code. This license was added under dubious legal ground and, at best, should've only been added once consent from all contributors whose code was incorporated was given.

* Adjust readme, add relicense.md

Author: Salanto

LICENSE.MIT

@@ -75,7 +75,8 @@ The logo (`logo.png` and `logo.ico`) was designed by Lucas Carbí. The character
 
 ## Copyright
 
-The project is dual-licensed; you are free to copy, modify and distribute AO2 under the GPLv3 or the MIT license.
+The project is licensed under the GPLv3 license.
+Code that was released under the MIT/GPLv3 dual-license can be found [here](https://github.com/AttorneyOnline/AO2-Client/tree/last-dual-licensed).
 
 Copyright (c) 2016-2018 OmniTroid
 

README.md

@@ -0,0 +1,38 @@
+# License Change Notice
+
+## What Changed
+
+This project was previously dual-licensed under both MIT and GPLv3 licenses.
+The project is now exclusively licensed under the [GPLv3](https://www.gnu.org/licenses/gpl-3.0.en.html) license.
+
+**Effective for any commit after `4db979187386326df64b9359b8de5e90468f7fc3`, tagged as `last-dual-licensed`, all new contributions and releases are licensed exclusively under GPLv3.**
+
+## Impact on Existing Forks
+
+If you forked this repository when it was available under the MIT license:
+
+### Your Existing Fork
+- **You can continue using your existing fork under the MIT license** for any code that was committed before this license change
+- The MIT license you received for that specific version remains valid and irrevocable
+- You are not required to change the license of your existing fork
+
+### Pulling New Changes
+If you pull new commits from our repository into your MIT-licensed fork:
+
+- **New code from us will be GPLv3-licensed**
+- This creates a mixed-license situation in your repository
+- Any binary built from a mixed tree must comply with GPL v3 for the whole work unless you can isolate the MIT parts as separate, independent modules
+
+### Your Options
+
+1. **Keep your current fork as-is** under MIT license and stop pulling our updates
+2. **Accept the GPLv3 license** and continue receiving updates from upstream
+3. **Fork from the last MIT commit** and maintain your own separate development
+
+## Recommendations
+
+- Review the differences between MIT and GPLv3 licenses to understand the implications
+- Consider whether GPLv3's copyleft requirements are compatible with your project goals
+
+---
+**Note**: This notice does not constitute legal advice. Please consult with qualified legal counsel for advice specific to your situation.