Add the MIT LICENSE file

The README has claimed MIT since the very first commit without ever
shipping the actual license text — a bare "MIT" line grants nobody
anything. Land the full SPDX wording plus a copyright notice and link
it from the README's License section.

LICENSE

@@ -0,0 +1,18 @@
+MIT License
+
+Copyright (c) 2026 Paul Klumpp
+
+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

@@ -95,4 +95,4 @@ Chatto v0.4.19. Protocol version 1 (binary protobuf over WebSocket).
 
 ## License
 
-MIT
+MIT — see [LICENSE](LICENSE).